Because a few movies did "tragic villain" decently, audiences liked the switch-up, and the talentless hacks took it to mean that ever villain has to be "complex".

self-interested megalomaniacs

So billionaires?

The villain is the protagonist in most superhero movies, the superheroes are generally the ones maintaining a vague status quo in the antagonist role

Nolan's batman movies did really well and they had more "grounded" villains (well, so long as you ignore scarecrow, ras al ghul, bane, talia al ghul...), so this one tried to follow the same trend. It also apparently brought in Nolan's tory politics and utter contempt for the poor. Remember when Bane stages a people's revolution in Gotham only to then decide he's going to nuke the city for no reason?

I was gonna say that they did make Lex Luthor a billionaire tech-bro asshole in the Superman movie, but then I also remembered that his opposition to Superman was out of a commitment to a bizarre interpretation of reddit-atheism, and didn't have anything to do with competing visions of the social good.

To some modern treat enjoyers, having villains actually just be willingly evil assholes would be "cookie cutter" and make the story seem "one dimensional."