To get a job, you have to have a college degree and a good GPA. To get that, you have to get good grades in college -- which means doing the homework and taking the tests. To even get into a decent college you have to have a good GPA in high school -- which, again, means doing the homework and taking the tests. Most companies don't care if you actually have the skillset, or if any of those homework assignments actually taught you anything, you need the credentials to get in the door to show it.

Grades aren't as big of a deal if you're going to be your own boss or start a business or found a startup, but in typical corporate America it's the end-all-be-all until you've got years of experience to show instead.