Libraries offer tight integration with the language. A generic service that provides a HTTP API doesn't.

If you are currently using a library that operates synchronously on in-memory objects and offers an idiomatic API in the language of your choice, switching to a web service may feel like a bucket of cold water because suddenly you're dealing with an async-only interface that sits behind a slow socket and requires serializing everything to a lowest common denominator API. It's a huge tradeoff that requires serious justification.

Back in 1990, there was an industry standard called CORBA that attempted to turn libraries into services: https://en.wikipedia.org/wiki/Common_Object_Request_Broker_A...

There's a reason why we're not using any CORBA-based software. (Well, the GNOME desktop was based on it for some time, but they gave up eventually.)