A lot of networks were designed with ipv4 and NAT in mind. There really isnβt a cost benefit to migrate all your DHCP scopes, VLANs, Subnets, and firewall rules to IPv6 and then also migrate 1000βs of endpoints to it.
Much cheaper to just disable ipv6 entirely on the internal network (to prevent attacks using a rogue dhcpv6 server etc) and only use ipv6 on your WAN connections if you have to use it.