The obvious advantage is that C++ compilers are much more standard compliant and better at type safety too, and allows for additional things like constant numbers instead of defines.
Obviously, C is very old and C++ can do all C can and more, and with the very same speed and efficiency.
So I don't see why we use C to write C code and not try to use C++ to write it instead so we can get rid of C.

This I what I don't get.