Except that URL's (visible pages) often don't map 1-1 to "content", and while they originally were "supposed" to, reality is far more complicated than that.

People like to be able to browse pages in an "intuitive" way. This means often combining multiple pieces of content onto a single page, or splitting up a single piece of content onto multiple pages, or often both.

In the real world, URL's are human-friendly pages which generally try to hit a sweet spot between too little and too much visible information, not unique identifiers of logical content.

Which is exactly why API's are useful -- they are designed around accessing logical content. But this is not what normal human-readable webpages are generally designed for, and rightfully so. They serve different purposes, and insisting that they should be the same is just silly.