Because, I don't think it's that easy.

Mainly because even assume you take outter wall, I'm willing to guarentee there will be a little town square, center, whatever we will have to hold again for two minutes and as comp retreats they'll fall back to that.

Further I would like to think that while you can take first wall, if you have to take it down by layers that probably means it's not interconnected. I am assuming though there is prep time so the key is stationing correct units. Also you can always do like with mideval and send units out up to walls, they just kind of slow to get up there.