Why must a school bus driver specifically watch the mirrors during a turn, even after the front of the bus has already cleared an obstacle?

AAMVA CDL Manual, Section 10.7.4 (Tail Swing), p. 10-11

New CDL candidates often assume that once the front of a long vehicle has passed an obstacle, the vehicle is clear of it. With a school bus, the rear overhang can still swing into that same space during the turn. Understanding this mechanism, not just memorizing the three-foot figure, is what makes the mirror-checking habit make sense in real driving.