I think it is because sapping (as it was in RTW) had no counter, even sallying out cavalry would have no use.
If you payed the buildpoints for it, then the wall would go down and there was nothing the enemy could do about it, 100% safe, 100% succesrate.

To make sapping a fun gamemechanic, there would have to be some form of counter to it, such as countermining.

I'm glad it isnt in in its RTW-incarnation tbh.

Now what I DO miss is the boiling oil from the gatehouses, right now it is far too easy to just smash a gate and just pour all your cav through it.