Shady grading system in college?

Say you have over 100 credit hours in college..all passing grades(mainly A's and B's), but have one Fail. Why do F's pull you down so much, it's like no matter how many A's or B's, you can't outweigh that one F. Is it nothing more than shady schools wanting to get students to retake the class to pay the money. How do the calculate GPA. The whole system seems shady.