Well, of course, there is ONE reason: Having to use old compilers or old standards for other reasons.

I imagine a lot of people are in that place with C++. Personally I've followed C++ for a long time as a hobby and it seems like it's finally getting to where it wants to be. But I'll talk to someone who works writing C++ and the idea of being able to use C++17 is a distant dream.

For Open Source developers and hobbyists though, you might as well take advantage of whatever features Clang+GCC+MSVC offer. I wish someone would standardize `#pragma once` so I don't have to feel guilty using it.