Merit badges and achievements are boolean: you don't have them, then you do a thing, and suddenly you do.

Learning something tends to be a gradual uphill climb: through practice and repetition you slowly become better. There are very few inherent concrete goals or thresholds, and it's generally fairly difficult to notice yourself improving because it takes place over a longer time period.

I struggle with perfectionism, which means while I enjoy learning I'm never hugely satisfied with the outcome. You can always bake a better cake, write better code, or whatever. But when you get that 'ding' of an achievement there's nothing even the most annoying bit of my head can argue about: I did the thing; there's no way to improve on it further. Annoyingly, that's far more satisfying for me.