True, but there was a time when FORTRAN was the most portable language, which meant that it certainly did get use beyond math. I've seen compilers and assemblers distributed as FORTRAN source.

And writing encoding-agnostic string library is harder than we think, I guess.

Notice that the first release of FORTRAN I is earlier than standardization of ASCII. FORTRAN was initially born for IBM 704 then ported to huge variety of machine architectures. So there's no guarantee on internal string representation; It might be EBCDIC, ASCII, or other ancient, machine specific encodings.