I didn't know that, is that universal or only for math? It definitely makes sense for math as you can get close and easily guess. So you can have no idea what you're doing and still get a 70% or so.
When discussing the multiple choice format between students, we've always thought that it's way easier to get the guess the right answer between choices than to show how you got there.
It's even worse outside of math classes, say history, and you're asked "when did $COUNTRY get its independence", obviously, it's much easier when you have a list of choices to choose from.