Taking off from a farther base slows the rate of replenishment of friendly fighters. Eventually the bad guys get the advantage and totally shellack the base and it falls.
In the fight we were in, the closest friendly base was 2 sectors away. The base fell because defenders couldn't get back into fight fast enough or survive long enough to make a good defense. It wouldn't be a big deal if it were humans organizing and coming up with overwhelming force. Its the fact that its robo-gunners mindlessly lobbing shells into a space. With 3 there, even the random rate of hits resulted in most of the defenders being toast.
Successful base defense depends on a triangle of numbers, skill, aircraft quality. Have all three, success is assured. With 2 of the three you can be successful at least part of the time. One of the three and you are losing.
We could let the robots completely play the game but they don't enjoy it as much
. Come to think of it, they don't pay for the experience either
.