There's plenty of reasons for it, but here are just a few.
A) You're playing multiplayer games, which are over saturated with 'competitive' type players and underrepresented with other types of players, such as story focused or exploration focused or creatively focused players. Those kinds of players tend to be far more easy going, and they are also too busy playing single player games to cool down the atmosphere in multiplayer games.
B) Antisocial parasitic retards are really fucking loud and obnoxious. They are constantly screaming and broadcasting their rage. More chill and relaxed players tend to stay quiet and just play the game. When I play a multiplayer game with a server of, say, 24 people, I generally won't find 50% assholes- what I'll find are 3 players that are public but chill, 3 players that are screeching their raging spergy assholishness, and 18 players that just keep to themselves.
C) Video games are easily accessible to children, and people who have never really emotionally matured past the state of children, for whatever reason. Children aren't really capable of separating themselves from a product, and will invest their ego and self worth into it. If someone is just kind of generally a failure that spends 100% of their available time playing video games instead of growing or maturing, then they'll continue to act like children with an inability to separate themselves from the game they're playing, which leads to seething hate and rage whenever any one of a million things causes dissonance. The server having lots of either manchildren or actual children will lead to a lot of screeching and sperging out.
D) Antisocial, antagonistic and socially parasitic attitudes are actually celebrated by large segments of capitalist society. See all the people who thought that "Greed is good" was meant as an inspirational quote. Cooperation is bad, dominating others is good. Kick and claw and fight your way to the top of the heap, then stomp on the face of everyone below you so that they can't climb up and dethrone you. These are seen as positive behaviours and encouraged, so why wouldn't that translate into video games as well?