Nvidia wants IP. They do not want to buy foundries. It's too volatile and dependent on government subsidies while also being well outside their core competencies. If their products don't sell as a fabless company, they don't see growth and lose on the manufacturing cost and stop placing orders. If their products don't sell and their chip fabs run idle, they lose a shitton of money on not just the above but also the cost of maintaining the fabs. Not only is there no guarantee that Nvidia would be able to run the foundries better, it's actually quite likely they'd run them worse.