I think it's partly this and partly the way players and developers think about dungeons more generally. In something like WoW or other MMOs people rarely go into dungeons without foreknowledge of the layout, mechanics, and expected rewards, to the point where not
already knowing the dungeon is considered poor etiquette. The developers reinforce this by making repeatedly running dungeons (regular or mythic+) a core part of the game, and in mythic+ you're expected to get it done as fast as possible to maximize rewards. If it's meant to be replayed over and over players expect to get through it reasonably quickly, as having to slog through an hour (or more) long dungeon gets old fast once you've already seen it. People aren't running the dungeons to explore them, they just want the reward at the end.
In open world games I think developers shy away from long dungeons and branching paths because they don't want players to be unprepared or have to make multiple trips back and forth to complete it. Bethesda games in particular focus on having one or two interesting things about a dungeon before wrapping up and handing the player their reward, rather than having them be balanced and well thought out challenges the player needs to overcome, since a player at any level with any combination of skills (no matter how useless or poorly built) is supposed to get through it. Also, that Morrowind comparison is unfair, as most of the dungeons in that game are straightforward and the end game dungeon is just a line. It'd be like comparing Blackreach in Skyrim to the cave outside Seyda Neen.