Warning this question may ruin Batman for you

[Blue22]

The notion that one man can single-handedly 'fix' a city by throwing around billions of dollars is a bit absurd.

Then why hasn't anyone been executed

Because, like everything in Gotham, the criminal justice system is fucked beyond repair. Anyone who would normally get the death penalty is, instead, labeled as insane and thrown into Arkham, where they wind up escaping two weeks later and repeating the process all over again.

At this point, Gotham and unfixable are pretty much synonymous. And, for the sake of the stories that they tell, it kinda has to be that way. The most Bruce can do, as both himself and as Batman, is put a band-aid on a gaping wound.