I'm imagining, but someone please correct me if I'm wrong, that this is related to the policy/standard/convention (?) that IPs have a single canonical name, so reverse and forward match. I've never really followed that strictly, though I've been aware of it and try to follow it as I can, because it makes management of DNS a little more sane.

Because of no SRV records for HTTP and no CNAME for apex records, you can't follow "one true name per IP" without giving an IP to every domain.