Not every server is useful, I guess. People used to use backdoored servers to send spam, but that's harder and harder to do from untrusted IP addresses.
Thinking about it, if you want secret information by definition only a tiny fraction of the networks out there are going to have it— anything most of them have wouldn’t be a secret anymore. So any attackers are going to focus the majority of their energy on exfiltrating data from the most likely/productive extreme minority of their infiltrated networks.