Mostly because there aren't many arm devices that target the popular linux use cases (desktops, laptops, servers). And even then servers don't generally use GPU

Mostly because there aren't many arm devices that target the popular linux use cases (desktops, laptops, servers). And even then servers don't generally use GPU's for anything other than AI. Nvidia on the other hand rules the laptop market for dGPUs and has a big share in Desktops. Secondly, unlike ARM, Nvidia goes out of it's way to make it's hardware not work with open drivers. Most ARM vendors generally ignore open source drivers. Nvidia on the other hand locks down the hardware with very insidious tricks (very low default clocks that can only be reclocked to their advertised speeds by a Nvidia signed driver) to specifically block any alternate open drivers. So even if the community is determined and funded to reverse engineer and make an alternative driver, they quite literally can't.