Sep said
Because you can't just "Swap out" AT-AT space to add additional fighters. An AT-AT hanger isn't designed to take TIE fighters and vice versa. Getting rid of the AT-ATs from the compliment just means no AT-ATs :P
Actually it can be configured to be. TIE fighters are stored on "Hangars," like, hung from a suspension, they can be put in there as storage. Plus, where the AT-AT is located has a lower hangar door to be released from. Though it can not be used as an actual hangar, they can still be stored inside. If you are not fond of this idea, I will just have the area converted into factories for armor and weapons. I already had other places reconfigured for that, but eh.