1) They dig to fast. Seeing sapping in real time would have taken months.

2) The only way sappers take casualties in RTW is if they're hit by an occasional arrow, however most of the time the sapping points are out of reach of the towers. Sapping in RTW is safe whereas in real life it was not. If they could defenders would sally out to wreck the sapping entrances, but in RTW when the battle starts you'd rarely have enough time to do that (see last point)
And in real life there's of course the danger that the shaft will collapse.

3) You have no way to chose where to sap, and towers cannot be destroyed by sapping at all. Only towers with deep foundations should be safe.