There are specific tactical battle maps that get picked. I don't believe any of them are randomly generated. And the map edit should allow you to make your own tactical maps.

I can't answer the why with any authority on the subject, but I think it's mostly a question of balance. By their nature, randomly generated maps aren't balanced, which is something that's generally required in a "battle". There are obviously some things that are meant to be a bit unbalancing, such as the defending being behind walls, but even then by hand it's possible to create maps that are balanced for their required roles.