Because when you design a Web API you're not designing a library, you're designing part of a distributed system.
Accessing code on a remote server is not like accessing code in a DLL, and never will be.
It's a computer asking for an action talking to a computer asking for an action.
Call them whatever you want, the concept is extremely simple and pretending it's anything more complicated is obfuscating the underlying simplicity of the intent of the programmer. i.e. I want the other computer to do something or the other computer to send this computer some information.
Even if RPC is a fundamentally, unambiguously simpler way to express that intent (and I think that's arguable), it does not meet the same technical requirements that REST does.