So...to speculate: I know that spam email is coming from an IP; I don't necessarily know that the IP is officially associated with a given domain. Sure, I could do a dig to see if the A record matches the IP, but blacklisting an entire domain is a pretty draconian step. I need my blacklist to be accurate, with a minimum of false positives, or its market value diminishes. Adding domains based on IP association is going to be more likely to induce false positives and increase the administrative overhead involved with my blacklist, even if that overhead is mostly automated through "get me off this blacklist" forms and the like. /speculation

As an aside, I've noticed that if a cloud provider recycles IPs (Digital Ocean) the chances of pulling a blacklisted IP off the heap is pretty good. At this point, even if all the server is doing is sending the occasional password reset email to a handful of internal staffers, we run the email through Mailgun. It just isn't worth dealing with the hassle of trying to get an IP off a blacklist.