You should read Fielding's thesis, but the short version is: when you're using a single endpoint, you're not really simplifying the communication -after all, your application still needs to do different things-, you're just replacing parts of the standard HTTP methods and server-sent URLs (which the client doesn't need to know beforehand) with custom/proprietary method names which the client needs to be coded against specifically.

Of course, this depends on whether you're actually following REST or not. If you're hardcoding endpoints all over your client applications, your architecture is not really RESTful.

Still, even if you do hardcode them, you still gain some advantages: for example, you can add a layer of caching by just sticking Varnish in the middle, which is not possible with RPC without custom code. That's the advantage of following the Uniform Interface constraint.