Because the whole point is to handle anything that needs Unicode support. A library that only manipulated Unicode strings would be incomplete if you still couldn't use Unicode in command line options, file names, etc.

I would recommend breaking them off into separate additional libraries. I don't need unicode for flags, so paying for it at compile and link time seems unwise. Or provide adapter classes that can be used over other frameworks. Just a suggestion.

That's what will happen until there's a defacto/standard library for this stuff. Languages like Python and Go have a wider base in the standard library. C++14 still only gives you platform dependent 'wide' strings, UTF-8 string literals, and UTF-8 conversion... which makes things awkward.