That's not entirely true. Many VPN providers and apps have settngs for custom DNS servers. In some cases, like NordVPN, it makes sense to use AdGuard DNS servers because NordVPN uses Google Analytics that gets blocked by AdGuard DNS servers.
On top of that, many VPN providers do not use any DNS encryption methods. It may seem like overkill to use DNS-over-HTTPS and/or DNS-over-TLS within VPN tunnel, but it isn't. Authorities, for example, are well aware of datacenters used by VPN providers. Once they establish exit node surveillance, they can re-direct traffic via DNS hijacking. That's not as uncommon as you'd think. Mass surveillance is a thing and governments consider VPN datacenters to be hideouts for criminals. With encrypted DNS queries, you'd at least be visiting real non-spoofed sites.
I don't know if you anyone with NordVPN tried to access Amazon.com on PC lately, but I did, and it directed me to a site blocked by my firewall. Why? My firewall blocked all reserved and private IP ranges (even within VPN tunnels). NordVPN DNS directed me to a 100.X.X.X IP (reserved range) for Amazon.com. It was some kind of a proxy. Other private DNS providers didn't do that and took me to actual Amazon.com hosted by Amazon.
Also, using private DNS server in Android settings is a good idea even for VPN users. It makes sure the initial DNS resolution of your VPN tunnel domains is encrypted.