C++ has value semantics, which means that values of user-defined types generally behave like values of built-in types. In this sense, copying is the only logical default. It's just how the language has been designed.

Things are different in Rust because of lifetimes and destructive moves. In this context, copying would be a bad default indeed.

> because who said that's even achievable, let alone cheap?

Nobody said that. The thing is that user-defined types can be anything from tiny and cheap to huge and expensive. A language has to pick one default and be consistent. You can complain one way or the other.