There's a difference between an API and a Platform.

An API has a clear boundary; as a user you interact with it in a single, clean, self-contained interface.

A platform has much less clear boundaries between it and your use case.

A platform is a messy thing that can absorb the complexity of your use case and model it.

A platform that is not possible to be used in messy ways won't be used widely, because real world use cases are inherently messy.

The things you optimize for when building an API vs a platform are different.

API: do your one task well with minimal fuss.

Platform: never say no to any use case (though they aren't necessarily easy to accomplish).

Platforms are extremely valuable because they empower users to solve classes of problems they couldn't otherwise... and every bit of incremental usage the customer does with the platform makes them stick to it slightly tighter.