It looked to me as if the zombies moved around a lot. They should have made better use of the good OP they had. Check out Charleston Heston in "Omega Man".

With such a good OP and vehicles inside an overhead garage door they could have staged a rescue/resupply when the hoards had moved away from the mall. Heston found out that you could slip out when they weren't looking and do what you needed to do if you kept a low profile.
They didn't have a good leader among them.