I would argue that you're drawing a false connection between being GPL and the benefits of open source code. GPL provides legal protections, but does not directly or indirectly incentivize writing open source code.
Having open source drivers grant Intel and AMD at least the following:
1. Free work on their drivers. Who wouldn't benefit from bug fixes and improvements to the graphics drivers of extremely popular graphics cards? In other words, it reduces cost.
2. Good marketing. There is only a very small group of people who are twisted enough to think that open source is a bad thing. There is a much larger group that see it as a positive thing (and then there's the group that doesn't care at all). In other words, it increases sales.
3. Greater ability to mold the Linux kernel to better accommodate their drivers and needs. Even though a company can send patches to the kernel that would help their closed-source drivers, if they only help closed-source drivers, they are very unlikely to be accepted. On the other hand, any sane change that would help an in-tree driver would be accepted without question. In other words, it increases flexibility.
None of this relates to GPL (the drivers are probably only GPL because GPL forces the in-tree drivers to be GPL), and the same logic applies to *BSD's. The reason we don't have good graphics drivers on BSD is likely because it takes a lot of effort to write one. If it isn't going to affect revenue, Intel and AMD are unlikely to care.
Having open source drivers grant Intel and AMD at least the following:
1. Free work on their drivers. Who wouldn't benefit from bug fixes and improvements to the graphics drivers of extremely popular graphics cards? In other words, it reduces cost.
2. Good marketing. There is only a very small group of people who are twisted enough to think that open source is a bad thing. There is a much larger group that see it as a positive thing (and then there's the group that doesn't care at all). In other words, it increases sales.
3. Greater ability to mold the Linux kernel to better accommodate their drivers and needs. Even though a company can send patches to the kernel that would help their closed-source drivers, if they only help closed-source drivers, they are very unlikely to be accepted. On the other hand, any sane change that would help an in-tree driver would be accepted without question. In other words, it increases flexibility.
None of this relates to GPL (the drivers are probably only GPL because GPL forces the in-tree drivers to be GPL), and the same logic applies to *BSD's. The reason we don't have good graphics drivers on BSD is likely because it takes a lot of effort to write one. If it isn't going to affect revenue, Intel and AMD are unlikely to care.