Why don't Bertolt and Reiner break the city walls?
The main reason is that wall is very thick, due to


   titans inside the wall


according to chapter 33 of manga.

Female Specie was able to breach outer layer, but to completely bust through wall, you need to get through inner layer.

Armored Titan and Colossus are focused on gates for same reasons - gates are weakest part of walls formation.