Transparently copyable heap-allocated object would are a recipe for introducing invisible performance issues, especially in generic code.

Rust requires types to explicitly opt-in to being implicitly copied, while C++ requires you to opt-out by deleting the copy-constructor.

Accidentally copying small structs on the stack is a minor performance problem. Copying an std::box<int> in a hot loop could cause heap fragmentation, lock contention and huge amounts of wasted memory due to heap alignment requirements (32 bytes on 64-bit arches).