@xahteiwi I've always assumed, rather uncharitably that people don't bother to learn ISO8601 (or RFC3339) and UTC due to one or more of ignorance, entitlement, or laziness. Either they don't realise how not doing this could impact others, they don't care, or can't be bothered enough to do anything about it.
I see this in the same way as I see other accessibility and inclusion accommodations.