My guess would be that a lot of coding is autodidactic, aka there is a lot of self-learning, trial and error, etc. This means you learn a lot of bad habits, or develop gaps that a formal training course would avoid.

So when you get to something that is complicated and has the potential for deep strategies and shortcuts for efficiency (e.g debugging), it's no surprise that experienced coders might want a refresher.

Impostor Syndrome may be a thing there, too.