Because Tim Berners Lee was trying to solve document management the HTTP protocol has a lot of features which are distinctly not useful certain modern problems.
If you are building an API for an application then concepts from HTTP like HTTP methods and HTTP error codes which were designed for documents will just make your APIs more complicated.
It makes more sense to make APIs which will work with most of the common HTTP methods like POST/GET/PUT/DELETE etc. - you can ignore the type of HTTP request and HTTP error code and just call the APIs without ceremony.