Because, fundamentally, their biggest competitor is Google. A company with a market cap north of 4 trillion. If OpenAI does not spend hundreds of billions of dollars on datacenters, people will migrate to Google, little by little, and OpenAI will became a new Netscape story. A good product eliminated by an incumbent with infinitely deep pockets.
> If OpenAI is incapable of building anything that can't be easily copied by a third party
They can build better models, but for that they need a lot of compute and that's where the billions go. These better models can't be easily copied by a third party, because that third party needs to also throw billions at the problem, and billions don't grow in trees.
People are gonna move to google anyway, because Google can keep the gravy train running for much much longer. OpenAI's business model is totally reckless while Google is a cash rich company.