Quite simply, because of emergent properties, and abstraction.
When you consider the way things interact, you suddenly get behaviour that is much more complex than the constituent parts are capable of in isolation. If I gave you a book and asked you to look at only the letters in isolation, would you be able to understand the book? Nope.
Chemistry is the study of the emergent properties of interactions of particles, this emergence presents behaviour so complex that you literally do require a whole new field of study to study it. Just as books are the emergent properties of the interactions of letters on a page. If you try to read a book by looking at the individual letters, you'll not only waste your time, but you'll also fail to understand the book because you're too busy looking at the letters to see the meaning on the page.
Physics is the branch of science that studies how matter and energy behave, in a general sense. To do so, physicists often use simplified versions of real-world systems.
For example, to predict where a ball is gonna fall once you kick it, you can think of that ball as a single point containing the entire mass of the ball. You can get pretty accurate results doing this. The problem is, sometimes you have to make your representation of the real world a little more complex in order to suffice your needs. If you kick a ball, it can start moving forward or it can spin (if you kick it on the side). Now you need to account for the shape of the ball. You have to take into consideration the different forces acting on it to predict where it's going to fall.
You can imagine that more complicated questions require more complicated models. For example, it's relatively easy to predict what happens when two bodies orbit each other, but as soon as you have three bodies, it becomes a great problem.
Now, take a look at what chemists study. Atoms can be fairly simple (as a hydrogen atom, with only one electron moving around a proton) or really, really complex. To predict the behaviour of a single atom you have to take into consideration how protons and electrons will atract each other. Also, you have neutrons, which have no electrical charge. Electrons move really fast, so you have to take into consideration relativistic effects (which explain, for example, why mercury is a liquid). There are a lot of issues when you try to simulate an atom using a computer.
But chemists rarely work with single atoms. They work with molecules, which can be a lot more complex.
I just made a test. My computer needed 26.7 seconds to tell the most stable conformation for a simple organic molecule (1,2,3,4,5,6-hexamethylcyclohexane). It's made out of 12 carbon atoms bonded with as many hydrogen atoms as possible.
I'm using a normal computer, working at full speed. Things like these would've been very difficult until not so long ago, but chemists already knew the result I obtained before computers even existed. How?
Chemists just work with different tools. A chemistry students learns about 1,3-diaxial interactions, and about the different hybridizations that a carbon atom can attain. They learn about conformations, and about equilibria, and about Gibb's free energy and its relation with spontaneity.
That's the key. As you work with more complex systems, you need different tools. If you want to study how energy and matter behave in simple systems (and not-so-simple ones, now that we have computers), physics is the right thing to use. If you move up in the complexity scale, chemistry works well for atoms and molecules. As atoms and molecules start to conform bigger things (like cells and living organisms) you have to move on to biology. When a lot of different tissues create organs (like the brain), you need to use neuroscience and psychology. When a lot of humans live in society, it's very difficult to predict what they will do, so you have to use sociology and economy. Of course, as you move up the complexity ladder, predictions become harder to make and while some physical theories can predict phenomena with an astonishing accuracy, anyone would be skeptical about an economist who said that they know exactly how a certain country is gonna develop through the next 10 years.
That's the key: different systems, different tools. Since physicists work with simpler systems, they can also apply mathematics more rigorously, and that's why physics has that fame of being 'so difficult'. It's not inherently difficult, it's just more mathematically approachable.https://www.quora.com/Is-chemistry-just-a-sub-field-of-physics-Why-do-scientists-separate-the-two
In theory everything is explainable in terms of fundamental forces and laws, but in practice, no. Remember the three-body problem in physics class? Relatively simple systems like that and double pendulums show chaotic behavior.
I am not sure if physics can explain turbulence yet.
Even if it were possible, description at such a detailed level is not always useful. Classical biology is the best science for describing the difference between a chicken and cow, not theoretical physics.https://www.quora.com/Can-all-sciences-be-reduced-to-physics
>I began my reply by saying that nobody denies the amazing success of theoretical physics in the last four hundred years. Nobody denies the truth of Einstein’s triumphant words: “The creative principle resides in mathematics. In a certain sense, therefore, I hold it true that pure thought can grasp reality, as the ancients dreamed.” It is true that the fundamental equations of physics are simple and beautiful, and that we have good reason to expect that the equations still to be discovered will be even more simple and beautiful. But the reduction of other sciences to physics does not work. Chemistry has its own concepts, not reducible to physics. Biology and neurology have their own concepts not reducible to physics or to chemistry. The way to understand a living cell or a living brain is not to consider it as a collection of atoms. Chemistry and biology and neurology will continue to advance and to make new fundamental discoveries, no matter what happens to physics. The territory of new sciences, outside the narrow domain of theoretical physics, will continue to expand.
>Theoretical science may be divided roughly into two parts, analytic and synthetic. Analytic science reduces complicated phenomena to their simpler component parts. Synthetic science builds up complicated structures from their simpler parts. Analytic science works downward to find the fundamental equations. Synthetic science works upward to find new and unexpected solutions. To understand the spectrum of an atom, you needed analytic science to give you Schrödinger’s equation. To understand a protein molecule or a brain, you need synthetic science to build a structure out of atoms or neurons. Greene was saying, only analytic science is worthy of the name of science. For him, synthetic science is nothing but practical problem solving. I said, on the contrary, good science requires a balance between analytic and synthetic tools, and synthetic science becomes more and more creative as our knowledge increases.
>Another reason why I believe science to be inexhaustible is Gödel’s theorem. The mathematician Kurt Gödel discovered and proved the theorem in 1931. The theorem says that given any finite set of rules for doing mathematics, there are undecidable statements, mathematical statements that cannot either be proved or disproved by using these rules. Gödel gave examples of undecidable statements that cannot be proved true or false using the normal rules of logic and arithmetic. His theorem implies that pure mathematics is inexhaustible. No matter how many problems we solve, there will always be other problems that cannot be solved within the existing rules. Now I claim that because of Gödel’s theorem, physics is inexhaustible too. The laws of physics are a finite set of rules, and include the rules for doing mathematics, so that Gödel’s theorem applies to them. The theorem implies that even within the domain of the basic equations of physics, our knowledge will always be incomplete.
Freeman Dyson May 13, 2004 issue https://archive.ph/2LvBv