I can't quite get behind the idea that Fort Irwin is completely overrun with zombies.

Helicopters make a lot of noise, which would draw in any and all zombies. Kalani mentioned that after coming in loud at LAX, they were swarmed and only 3 people escaped alive. If the uniformed individuals were in fact zombies, they would have come running. As it is, there's a long, nail-biting pause before we hear a single zombie approach the helicopter.