Nvidia had the chance to build its own AI software and chose not to. It was a good choice so far, better to sell shovels than go to the mines - but they still could go mining if the other miners start making their own shovels.
If I were Nvidia I would be hedging my bets a little. OpenAI looks like it's on shaky ground, it might not be around in a few years.
They do build their own software, though. They have a large body of stuff they make. My guess is that it’s done to stay current, inform design and performance, and to have something to sell enterprises along with the hardware; they have purposely not gone after large consumer markets with their model offerings as far as I can tell.
If I were Nvidia I would be hedging my bets a little. OpenAI looks like it's on shaky ground, it might not be around in a few years.