It's more that C++ is the only widely adopted fast language. Rust and D are the only real challengers there but D doesn't seem to have caught on and Rust's growth, while impressive, isn't up to widely-adopted language status yet.

Granted most things don't /need/ the raw throughput potential of C++, but if parts of your code really does need it then it tends to be easier to just make the whole thing in C++ than deal with bindings to other languages.

Only those that never used other languages besides C.