Why did they screw up the Percy Jackson movie so much?

so i know this question has been asked a lot, but i am just so outraged i will say it again. WHY DID THEY MESS UP THE PERCY JACKSON MOVIE SO MUCH!?!?!? I mean really, almost urgently they messed things up. So why!?!? the books were amazing. why did they change so much for no reason?!? i guess i understand it a little, but little things like percy not being in bording school and hades being like some weird demon thingy. its bull crap!!!