>Surely the pilot would notice at least one of the planes he'd be inevitably crashing into well before he started to land
In theory yes, but the risk is that the plane you crash into is not yet on the taxiway or that poor visibility or tunneled focus prevents you from noticing it.
You don't need to be moving quickly to have a significant accident. The deadliest aviation accident in history (583 fatalities) resulted when two planes collided on a taxiway (edit: actually on the runway) in dense fog [1].
Edit: as correctly pointed out below, the actual collision took place on the runway at significant speed.