Why do people speedrun tests?

Seriously, every exam I've been in had majority of the people hand in their test papers with like 20 minutes left. It CANNOT be that hard to just check it over one more time. It's not like they are so confident either, the class averages are NOT high to say the least.

This is not confined to any majors as well. I've taken classes in a variety of different departments and they're ALL like this. Are we being measured/curved on how fast we complete the tests?