Well there are models that Anthropic, OpenAI and co. have access to that they haven't provided public API's for, due to both safety, and what you've cited as the competitive advantage factor. (like Openai's IMO model, though it's debatable if it represented an early version of GPT 5.1/2/3 or something else)

https://sequoiacap.com/podcast/training-data-openai-imo/

The thing however is the labs are all in competition with each other. Even if OpenAI had some special model that could give them the ability to make their own Saas and products, it is more worth it for them to sell access to the API and use the profit to scale, because otherwise their competitors will pocket that money and scale faster.

This holds as long as the money from API access to the models is worth more than the comparative advantage a lab retains from not sharing it. Because there are multiple competing labs, the comparative advantage is small (if OpenAI kept GPT-5.X to themselves, people would just use Claude and Anthropic would become bigger, same with Google).

This however may not hold forever, it is just a phenomena of labs focusing more on heavily on their models with marginal product efforts.