Um... Aliens need their base as much as anyone. Their defenses are weak, though: if humans control the map and the aliens are pinned in their base, it's just a matter of time before the humans take down the acid tubes and destroy the base. Fortunately, aliens are usually in a position to control at least their half of the map, if not the entire map.
The reason human defenses are so strong is to stop the agressive stage 1 and 2 aliens from completely raping the human base and ending the game 5 minutes in. Not only does it prevent the alien team from winning by agression alone, but it also means that the entire team can leave the base for short periods of time (assuming there's a good turret layout). So, in a way it helps humans get out of the base, because they know that it can survive by itself for a while if the layout is good.
(of course, this all changes in s3, when no base is safe by itself)