The impression I get is FAST is trying to simplify the markup you see in for instance Bootstrap's documentation by trying to build standard components that Bootstrap themselves could use (or Material or their own Fluent stuff). The emphasis is on a lot of the little things in Bootstrap's documentation that doesn't either get copy/pasted or it gets copy and pasted and ignored/forgotten/left-to-bit-rot. Stuff like aria- tags for accessibility. Seems the idea here is that by baking them into web components that do most of the work for you they are less likely to be forgotten or left to bit-rot.

(I'm not a front end dev myself, but do enough full stack to think I know what is going on here.)