I think it's clear that many, if not most, are not arbitrary - which doesn't mean they are well supported; not everything is valid just because it has some reason behind it.
The definition of arbitrary is "having no reason behind it". Sure, judging whether a particular classification counts as "arbitrary" is hard because the definition of "arbitrary" is squishy. But what you just said amounts to:
It's not arbitrary. It might be arbitrary, but it's not arbitrary.
The definition of arbitrary is "having no reason behind it". Sure, judging whether a particular classification counts as "arbitrary" is hard because the definition of "arbitrary" is squishy. But what you just said amounts to:
It's not arbitrary. It might be arbitrary, but it's not arbitrary.