The fact that ARM has not made their Mali drivers open source is a joke. There's no excuse, same for Nvidia.
It's arguable in the case of ARM/Mali but sadly, there is for NVidia. Their driver are not merely to "communicate" with the hardware. The driver include a lot of hotfix and optimization, sometimes specific to one game/application. The driver give them a competitive advantage over their competitor. They will probably never released it for free.
Interestingly, amdgpu/Mesa performance seems to be higher for typical game rendering than AMD's proprietary stack. I suspect at some point in the future AMD will abandon their 'pro' proprietary stack entirely after realizing they should stick to being a hardware business.