Think of grades as practice paychecks... you might think that money would be enough to make the difference for you, but if you're that strongly disinclined to do things you disagree with, you may very well find that when you end up working 40+ hours a week doing the same b.s., you just can't bring yourself to do it. Happens to a lot of "intelligent" people.

Knowing that you can excell not just "when you really want to", but on a regular and consistent basis can make existence a lot easier, since "really wanting to" isn't something we can actually control over the long term.

But still, it doesn't make me dumb to be in 5 years of highschool when I can obviously do something about it

Sure. And whacking yourself with a hammer until you pass out doesn't make you dumb if you could stop at any time. Let me put it this way: who's dumber, the person who burns themselves on a hot stove but had no idea what it was, or the person who burns themselves knowing full well that it was hot and dangerous yet not having wanted to be burned?