3-point belts are made to be as quick and fool proof as possible so that people actually use them, and use them correctly. Importantly, they auto-retract so that they are worn in the proper position.

When you put on 5-point belts, you have to 4 times as many latches to operate, and you have to adjust all 5 belts -- and in the proper order. If you tighten the belts in the wrong order, they won't protect you properly. It is not intuitive -- I have a car with 5-points and the passengers who manage to figure out how to latch them almost universally tighten the belts wrong.

Edit to answer my own question: apparently they use spotters that radio in to them whether or not it's clear, and (at least in NASCAR) the cars don't have side mirrors but have an extra long rear view mirror.