Cuz people would always be saying "can I have one more day" There has to be a cut off at some point so that schools can start to sort applications. Plus...it keeps people for procrastinating too long... no "well, they said by the 31st, but it's ok if I take it after that..it'll only be a few days late". So, yes, it's a pain in the butt, but it's for your own good and so they can get the admissions process rolling!