Paul English (founder of Kayak, exited at $2 billion) said this in response to the exact comment you are making: "Kayak was just a thin layer above ITA, and ITA sold for $700m and Kayak sold for $2b. So don’t dismiss thin layers. The ultimate power is the app and UX."[0]

I would also add "distribution" to the "app and UX" part, but you can certainly build a valuable business upon an API "everyone else has access to" - plenty of companies out there that do that.

[0]https://twitter.com/englishpaulm/status/1623701758781558784