But Batman DOES uses his billions to help Gotham. The problem is that most of his rogues gallery is composed of high ranking rich mafia bosses, and the rest are completely insane individuals (or even sane people who just LIKE being evil for no reason like Scarecrow who literally gasses people until they die of fright just to study their reactions). You can't possibly think bribing the Joker with a billion dollars would stop him from bombing orphanages, gassing entire neighborhoods, or LITERALLY NUKING AN ENTIRE CITY.
Investing money into Gotham isn't gonna stop the Penguin or Black Mask from running protection rackets or bribing cops, it won't stop Poison Ivy from commiting literal ecoterrorism, and it won't stop Victor Zsasz from gutting people to death with knives, or Deadshot from assassinating people.
Oh, and another thing: the city is actually fucking cursed with evil black magic. That's not hyperbole. The city literally makes people evil.
Batman only is a vigilante because his work as Bruce Wayne is not enough.