The biggest crimes are committed behind a desk

Harvey Dent.
That is all.
I also personally believe that the villains portray the corrupt establishments.

Well it's not like Bruce Wayne does nothing all day...

Bruce Wayne and his billions (trillions) is the cause of the poverty which creates the crime that he fights. It's a self perpetuating cycle.

Doesn't he own a greedy corporation?

He does both tho

Harvey Dent, The Penguin, numerous big time drug lords that's have more control than the politicians......

Because it doesn't make for interesting stories

Bruce became Batman to enact revenge for the loss of his parents. Obvious crimes with clear good/bad guys and quick resolutions fit in with this temperament of single minded pursuit with a high effort to reward percentage. Crimes that take months or years of tedious drudgery to finally have enough evidence to convict someone are almost never as clear cut nor quick to resolve. In short the very reason he became a vigilante is also the reason why he doesn't target the corrupt politicians and greedy corporations.

Yes, why would a multi-billionaire fight the system that allowed him to become a multi-billionaire.