They won't start from scratch.
You can license ARM cores, scale up the design a bit and get to first pre-production units within a year or two.
OpenAI needs extremely large amounts of computing power and is almost 100% dependent on Nvidia to get it. That's not a good situation to be in. Even if they can't compete with Nvidia directly, building their own chips might be cheaper than paying Nvidia's profit margin.
Dell Precision, HP Probook for example. There are probably more, but these are the ones that I know of.