Having worked for a GPU company, it's not even about stealing IP. It's about giving ammo to your competitors for suing you. If you publish the source code to your drivers then let's say Nvidia can read through them and find a few similarities in your hardware to something they have patented, and then sue you for IP infringement. They can't do this without some evidence that there might be a similarity, but weak evidence is enough. So they basically send you a set of 100 patents that they say you violate, and then it's your job to spend your legal resources to show how you don't violate those patents. It might be easy to show this for say 90 of those patents, but there can be a few which require considerable work, and if you're a small company you don't have the resources to fight off lawsuits all the time. So open sourcing your drivers opens you up to a lot of potential attacks, with very little upside. More specifically, it opens up the companies that buy GPU IP from you to attacks. Nvidia on the other hand has the resources to keep throwing accusations at you and see what sticks and force you to burn cash on legal resources.