Its not even that. You think that making buildings is more complicated. Technically, yes they are. However they are connected to nothing else. Units on the otherhand are connected to anything. Want to test stats? You need units. Want to test recruitment? You need units? Want people to be motivated to work? You need units. Want to work campaign balance? You need units.

Ancient Buildings are nothing but aesthetic add-ons, but the units are necessary because they connect to everything else.

Foot