Most of these are Microsoft's fault: MS-DOS allows only 3 characters for the file extension, so the file extensions had to be abbreviated.

As for C++, I'd also blame Microsoft. The plus character (+) is a reserved character for MS-DOS, so the obvious extension ".c++" couldn't be used (nor could the case-sensitive ".C" extension). So people either toppled their plus signs (".c++" becomes ".cxx"), or replaced them by the first letter of "plus" (".c++" becomes ".cpp"), or treated them as a repetition sign (".c++" becomes ".cc").

C++ is the worst offender: .cpp, .cxx, .cc, .h, .hpp, .hh, etc.

I've even seen .c++/.h++ in the wild at least once.

Could be worse, could be uppercase .C/.H…