Not everyone has the option of using whatever popular library happens to be around. Some of us are stuck with whatever the standard gives us, and not always the most recent one either.
Yes, it is available everywhere, doesn't require an additional dependency on older compilers and not everyone hates iostreams, some of us are happy users since 1993 (my first C++ program).
The two you mentioned don't support streams. Streams make typing string concatenation code much more convenient than having to deal with nested parens.