Selling a GPU takes time and loses you money - and that's in PC case, because on a laptop you don't even have that option.

Moreover, if you have an NVIDIA GPU, you most likely need a decent GPU for something - high-end video games, 3D modelling work, GPU-accelerated computations, etc. Linux distros are, frankly, not much different for one another - they're pretty interchangeable for almost all tasks I can think of as a user/developer. GPUs, on the other hand, are a bit less interchangeable between vendors. You really have only two players to choose from (Intel GPUs don't count for anything serious) - so if both suddenly become incompatibile with a given Linux distro, then it's obviously bye bye to that Linux distro.