In no small part this is because parsing long opts can be significantly more complicated than short ones. It is by contrast very easy to parse short, single-letter flags. Not saying it's not worth doing, but there is a reason this has not been standardized.
Parsing long opts is trivial; just determine if the string exactly matches. E.g., "--version" only matches "--version". Short opts is what takes slightly more effort, because you have to accept both "-ld" and "-l -d". We already handle the "hard" case, we just need to support the easy case.
