It's entirely your own doing if you don't get good grades IMO unless you have a legitimate disability. But some people were idiots when they were younger, or had other (questionable) priorities. I've also heard really sad life situations where teens wind up essentially having to take care of their siblings and the household due to dysfunctional parents or other stuff.
Because yeah, if your pubescent, immature brain decides that you have better things to do than schoolwork, you've really screwed the pooch and lost your shot.
Can't well go back to highschool and try over, haha.
(It worked out okay for me, I eventually got a GED and that's it. Never finished highschool or went to college, but programmed for fun as a kid and so managed to worm my way into software eventually. It probably would not have worked out in about a billion other scenarios, RIP)
C's get degrees, but the marginal difference between a C and and A wasn't all that much time.