Web servers serving malware are exploited in drive-by scanning; find a vuln in a webapp, drop your malware. It doesn't even take exploiting the system itself, and generally does not affect the web server at all. Taking over a DNS server would take much more work to pwn first, and then require reconfiguring the DNS server. Much more difficult.

Fetching a user-supplied URL is not enough to exploit remotely. You have to exploit the target's DNS resolver, because you have to feed it invalid or impossible records. All existing DNS resolvers will reject these because they break RFC.

It would be much easier to exploit a web app and drop your payload and exploit it locally, which is what everyone currently does to pwn servers with rootkits.