It's probably because the mesh networks work pretty well. Well enough that most people won't bother with the process of chopping up walls and wiring the house. The cost and the hassle.OK, I had to look that up, apparently gig+ is 1.2 gig - so there are >1gig options now (yay, marketing).
My main point(s) still hold - you don't really need much over 400, and the speed bottlenecks are mostly going to be in your house. The boosters (?) you have are either wifi extenders (yuck) or mesh extenders (better), depending on what the base router is.
If you were willing to do (or have done) a little drywall cutting, wire fishing and patching, I'd guess that you could get massively better internet speeds throughout your house. I'm always surprised that there aren't more AV/internet contractors that do that kind of work, it's such a common problem. Perhaps it's not profitable.
And you are correct. Extenders are a disaster and should never be used. Always mesh.