The open source drivers have frequently been written by volunteers, or professionals who do it as only a small part of their job, have had to spend more time and effort reverse-engineering rather than having documentation and the ability to talk to hardware engineers, and are perpetually at least a few months to a few years behind the proprietary developers who had access to all of this before the hardware even came out.

It's impressive what the open source drivers have managed to accomplish, and they are improving, but they are severely handicapped compared to the proprietary drivers.

This excludes, of course, Intel, but Intel focuses on lower end integrated graphics rather than high-end discrete graphics, so it's not quite comparable.