In a large organization it will take months to track down all this stuff to make sure a subdomain should or should not be there, pointing at a domain that should or should not be there.
Nobody will risk taking anything down with multi-million dollar advertising campaigns potentially riding on each one. If you're not familiar with how these campaigns work, they work like hot shit: they pay everything in advance and then put together all the technical details. Sometimes literally the night before the campaign is supposed to begin.
So what you see now in DNS may be obsolete, or it may be valid, or it may be from an upcoming campaign. Gotta dig through contracts and crawl the corporate structure to figure it out.
Also, there's no big enough incentive to fix this. Spam for third parties? Eh, fuck 'em. Until it grows into something bad enough for the FBI to get involved they won't care.