They are very expensive.
So, please, it is very hard to get armies of so expensive units in large amount even by middle game. And, in the late game, nothing should limit army compositions. It is hard-earned gratification.
And, by the way, they sold D.L.C. to many gamers, including me.
So, just like with confederations for Nakai, you can change game in a more pro-freedom of gamers side. Thank you.