Because that only protects you from a small subset of possible threats that end-to-end encryption protects you from like DNS hijacking and any MITM-type scenario.
Sticking it on a VLAN only controls access, not data secrecy.
Broadcasting internal IPs on public DNS records is also a suboptimal approach that leaks information to the public. Local devices should be routed over layer 2.
Broadcasting internal IPs on public DNS records is also a suboptimal approach that leaks information to the public. Local devices should be routed over layer 2.