Game developers might like to see clean driver source but they don't get to choose what kind of GPU their customers have already bought. And 99% of gamers are not going to choose their GPU based on Linux drivers. So nobody has any leverage and vendors have no incentive to change.

Meanwhile thousands of universities and institutions are each going to be looking for 25,000 GPUs and they can choose what brand they buy based on what makes their internal developers happy. Hosts like Amazon and Google are each going to be buying millions of GPUs, and having better and more transparent drivers so they can more easily e.g. improve power consumption by a small percentage, can save them a million dollars/year in electricity.

Someone like Google could come to each vendor and say "first to have mainline kernel drivers gets all our business" at any point. Or the same result in the other order; once there are clean drivers third parties are more likely to make power consumption and performance improvements that give AMD the edge when the major customers crunch the numbers.

There is a significant competitive advantage in it for AMD to get this right.