This is done so you can use std::move to take ownership of the allocated memory in these objects rather than do a new allocation. Passing by value rather than rvalue reference let's your function be more flexible at the call site. You can pass an rvalue/move or just make a copy at the call site which means the caller (who actually knows if a copy or a move is more appropriate) gets to control how the memory gets allocated.
An unnecessary memory allocation is much more of a performance hit than suboptimal calling convention.
The vanilla interface often allows what's right depending on the context (e.g., if an unnamed return value is passed in, it can be move constructed, otherwise it can be copied). I saw some code bases provide an override for a const ref and a normal type.
Wouldn't this be very annoying to work with, because now you have to explicitly move or copy the string whenever you want to construct one of these objects?
It's kinda what Rust forces you to do, except that std::move is implied. Anything taken by value is equivalent to taking by && unless the type is explicitly marked as Copy (i.e. it can be trivially copied and the copies are implicit).
But yeah, in a c++ codebase, good modern practices are often verbose and clunky.
An unnecessary memory allocation is much more of a performance hit than suboptimal calling convention.