Canonically he does contribute to a lot of charities and organizations and such in addition to being Batman. There's got to be a bit of a 2-pronged approach, if he contributes everything to charities, there wouldn't be anything leftover to be Batman, and while building a better Gotham for tomorrow is the greater goal, they also need someone who can stop the joker today.

And in a city where crime and corruption is as deep-rooted as it is in Gotham, you need to be careful about where your money is going, because if you start throwing money at random charities there's a good chance that somewhere down the line a lot of it might end up getting funneled into Carmine Falcone's (or some other gangster) pockets. And even if the charity is totally on the up-and-up, if the crooks get wind of how much money is moving through there, they're going to try to worm their way in and get a slice of the pie. So unless you want to go all Ra's Al Ghul and burn Gotham to the ground and start again, Bruce would have to be very mindful of how much money he was giving where.