With the amount of money nvidia have they should invest in making their own fabrication plant.
No chance, they can't just jump in the fabrication game like that. It takes massive amounts of R&D and investments. Best bet would for nVidia to buy out a fabrication company but even then this wouldn't make much sense from a business point of view as there isn't gong to be much return for nVidia doing this. Might as well let the Fab big boys do what they do. nVidia are better off spending more time and money in R&D to design and develop GPU archs.
I can see what your getting at but silicon fabrication is a very competitive business so nVidia are better off staying a customer rather than a supplier lol.