Grades have been the way for schools to measure students and identify if they understand something or not, assumably.
It’s not an effective way to know if a student have mastered a dance move, made a video game, created a campaign, or learned wilderness survival skills.
If the only way we know someone has learned something is by doing the actual work, showing it to the others, and creating a change, how could we measure that? And what would be appropriate things to measure?
Grades motivate some students to study hard and become the top in their class. But for kids that choose to do their own projects, grades don't work the same. The motivation lies in their interests, and the only measurement that matters is getting to the places they have wanted to go all along.
Different students, different measurement.