Not necessarily GPL violations (there are cases of those too, but the bulk of it is not). It's just a matter of nobody having cleaned up the drivers enough to get them submitted for upstream inclusion. It can be quite a bit of work to do, and the next generation of product is sitting there waiting on the same people to make that work instead.

You can usually download and diff the source tarballs. Some vendors keep git repos so you can see changelogs as well. There's usually a lot of noise in there though, lots of various imported vendor drivers that duplicate things, firmware files in hexdump format, etc. It's pretty common to see diffs of millions of lines.