What exactly does it mean for someone like Alexander to “found a city”? Particularly in light of the fact that he was in a middle of a ten year, continent spanning campaign? A city needs building, inhabitants, commerce, etc. all of which takes a while to build and establish. Why would people just move to a random spot that he declared to be Alexandria on the X? What would be there on the site of those cities already and what did he add? Why did he even feel the need to establish new cities, instead of taking over the existing ones?
It's worth noting, as /u/Daeres does in this answer, that most of Alexander's foundations were not really of new cities outright, but either simply 'refoundations' of existing cities, or more often military garrisons that then became more substantial cities under the Successors, whose priorities and interests were somewhat different.