They do because they are more worried about how they do and can't really do work at home because they get home late at night when you do basketball. And they are tired and they need to take a shower and eat and do their chores if they have any.
Athletes, especially in college, get special treatment with tutors, easy classes and professors/teachers that are willing to look the other way when assigning grades. On the high school level, athletes have to take the same classes as everyone else and graduate before they get to college. One person I went to high school with played football and yet went to the Air Force Academy and was a top 10 student. Student athletes don't always perform poorly in school, but their path to graduation is sometimes different than those who focus solely on academics.