I can tell you from first hand experience that it is extremely difficult to ensure consistent, reliable ETL of JSON payloads between thousands of services.
APIs are unreliable. Schemas change with no warning. Data you expect might be missing and data you don’t expect might show up. You would be surprised how often things don’t work as you’d expect them to.
Zapier does have thousands of integrations, and some hundreds of them (the oldest or most widely used) are maintained internally for the best user experience.
These days though, many more are maintained by the API providers themselves, since it benefits them to provide a no-code solution for their users to link their API to thousands of others - a service provider can only build and maintain so many direct integrations themselves. https://zapier.com/platform
I have always thought it would be useful if API providers were able to provide the javascript bundle that Zapier runs under the hood on the dev platform alongside their API at a pre-defined route, so any runner (Zapier, Tray, MS Flow/Power Automate) could consume the API regardless of location (SaaS automation, desktop client, etc). I believe there's even an automation provider that allows you to "one-click" import the integration you've built from Zapier to their platform (the name escapes me at the moment, my apologies).
If the API provider is already maintaining the integration, it's a matter of automation providers moving towards a standard for how to express integrations as the ecosystem matures.
APIs are unreliable. Schemas change with no warning. Data you expect might be missing and data you don’t expect might show up. You would be surprised how often things don’t work as you’d expect them to.