Possibly because a) many use slingers, and b) no one takes the time to actually move up units within range of the townsquare to clear it or drive them into attacking out of the square.

I did do that as Baktria, though, with my 10 units of thanvare and spearmen. Clears the townsquare, but you get them all charging at you. My assaults take up to an hour and a half just so I get it right and rest my troops between engagements fighting their way up to the town square.