I guess it's easier to make a shovel than it is to make a GPU, and it's easier to do industrial-scale crypto mining for a company that already owns a datacenter than it is to build an industrial earth processing operation. If NVidia wanted to mine a lot of coins, they no doubt have a few employees that could get that started. But it doesn't seem like they want to, perhaps because it's not a sustainable business or perhaps because their customers are already mad at them and they don't want to make it worse. (Though it's not really their fault that customers are mad at them; they're actually mad that someone invented cryptocurrency, which is not NVidia's fault.)