Why not? They seem to be a lot of leeway before any specific company will find it cheaper to design their own chips, or even to move to AMD (ROCm is not as well supported).
Perhaps someone like OpenAI has both the expertise and incentive to do so, but not many others.
So if you think that maybe OpenAI has other options if NVDA increases their prices why do you think that one of the other big names ( MSFT,GOOG,IBM,TSLA,AMD,INTC,Facebook ) also cannot do the same thing?
I guess for the same reason most of them keep buying from Intel - their market position allows them to pass on the cost to their customers, so it's not worth the distraction.
OpenAI is more of a "one-(very impressive)-trick-pony", so they have a stronger incentive.
Perhaps someone like OpenAI has both the expertise and incentive to do so, but not many others.