Wireless doesn't really have the bandwidth for anything more dense than a suburb (and even that's probably really pushing it). Approaches like microwave are also less reliable due to LoS/weather problems.

2.4Ghz is noisy in cities/apartments, 5Ghz doesn't penetrate well.

All that on top of the fact that there's still not really a standard, easily set up protocol for that kind of mesh ISP arrangement.