If you think about it, it makes sense. We have gunboats now and we have not developed space travel yet. Light infantry units weight much less than armored units, and the atmosphere may not be good for the ICE that tanks need. What the game needs is better late round aliens. This should come through the quest feature. If you tame the aliens in the quest some will escape and inject their genes into the environment. You will get a note the the AI have done so if you go Purity.