Use it or lose it. Most people have no reason to need knowledge of trigonometry, so even if they’re taught it they quickly forget it.

I never really learned trigonometry until I started doing game programming in my spare time when suddenly that knowledge and linear algebra became necessary to understand. They only way I learned it was by needing to know it.

In fact, I regularly forget knowledge I don’t need to know. The stuff I do need to know remains fresh in my mind.