You are probably missing the point with your questions. They're roughly equivalent to asking two people playing chess why they spend so much of their time staring at the same sixty-four monochrome squares, pushing around similar-looking pieces for no other purpose but to surround one specific enemy piece with your own pieces.
The answer is: Because people are having fun with Dota. Because Dota is a series of interesting decisions, to appropriate a Sid Meier quote, and because the little intellectual victories you achieve by predicting your enemies' decision and counteracting them with a decision of your own are really satisfying. Of course, there are far more moving pieces than Chess, not all of them balanced. But that, I believe, is an essential part of why people play Dota.
The second reason is the feeling of power. Yes, it's a rather base thing, but if you are kitted with a full build, achieved through a series of correct decisions, then you're going to feel powerful. Then all the little intellectual victories add up to become an unstoppable avalanche of death (should you play the correct character)
The third reason is probably masochism.
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Take in mind that this is the perspective of a person who doesn't even like Dota as much and sees it as a game with much unnecessary padding and questionable design decisions. I'd place other games as the pinnacle of multiplayer gaming, the thing is that those are usually niche at best. A critical part of Dota's success is that it's really not that difficult to play altogether, if you aren't really serious. You can always faceroll something, and blame your teammates if things go south.