That’s not usually how standards are supposed to work — a POSIX operating system should not reimplement Linux, it should only have to reimplement what’s portable to existing systems.
Except POSIX has historically always been the intersection of the major Unix vendors. Which is why its always been the lowest common denominator, and is largely crap as a result.
Of course to a certain extend all non defacto-standards suffer from this because invariably one group or the other doesn't want to rework their system to fit the standard so they NAK it.
POSIX does simplify things, diverging from historical practice, when it’s reasonably the only way to standardize stuff that arguably should be standardized. And the vast majority of scripts are easily converted to POSIX-compliant shell script anyways — it’s certainly not “largely crap”.
Of course to a certain extend all non defacto-standards suffer from this because invariably one group or the other doesn't want to rework their system to fit the standard so they NAK it.