Most nameservers disallow zone transers (requesting the full zone file), preventing this. Additionally it's more and more common for IPs to have a fixed generic

Most nameservers disallow zone transers (requesting the full zone file), preventing this. Additionally it's more and more common for IPs to have a fixed generic PTR that just describes the IP instead of the host, and for a lot of hosts to not even have dns records. (My last webapp never bothered with dns for the dozens of ephemeral webserver instances).