It wasn't developers who designed C++'s template model which requires generic code to be fully defined in header files.
Inheriting C's textual include file based "module" system and then bolting compile-time specialized generics is a choice the C++ committee made, not C++ users. It was probably the right choice given C++'s many very difficult constraints, but that's what directly leads to huge compile times, not dumb C++ users.
C++20 finally standardized modules. Whether they will improve things significantly is still anyone guess.