Probably because they feel guilty about not studying so much, and want to make sure you don't do it. In this way if they screw up they'll be sure that you keep them company, instead of outpacing them.

This is a very common attitude, and will only breed mediocrity, if you let it influence you.

On the flip side, make sure to be social, when it's time to be social, and specially avoid continuously canceling stuff using your studies as excuses. Be smart and provide alternatives that fit your working patterns.