You might have been right at one point in time, but respectfully you're not right today. Today's modern GPU implementations -- especially on mobile because the APIs we support are so (intentionally) vague and encode a system with a wide spectrum of possible implementations -- are so complex and hide a lot of implementation specific secrets that other vendors would love to freely discover in source code.
Discovering the secret sauce is the very embodiment of my job for my employer, and I make a great and exciting living figuring out our competitors' myriad secrets, a lot of which is controlled entirely by the driver.
I could write an entire book about why there's potentially very valuable competitive data enshrined in GPU driver code. Some obvious examples: execution resources and scheduler details (including full software schedulers in many cases), register allocation strategies, optimising shader compilers, hardware bug workarounds, power management limitations, clocking schemes, API compliance workarounds; all highly valuable competitive data that would be plain as day if GPU driver source were available.