> One particular thing I’m puzzled by is the lack of a standard file extension accepted by the major compilers.
There is no standard file extension for header files either, or C++ source files. It would be _more_ puzzling if we'd gone against the grain and standardized a file extension! Especially as the C++ standard doesn't require an implementation to even support "file extensions", as the naming of source files on a file system is out of scope for the C++ standard.