The issue is POSIX standardizing legacy stuff like shells, thereby tempting people to write "portable" software, leading these technologies to ossify and stick with us for half a century and counting. Someone comes along and builds something better but gets threatened for not following "the UNIX way".
This is a very good point. I wonder how hard it would be to get POSIX to standardise a scripting language that isn't awful.
Probably never going to happen. There is a dearth of good scripting languages, and I would imagine any POSIX committee is like 98% greybeard naysayers who think 70s Unix was the pinnacle of computing.