And a subdomain requires negotiating with someone in the bowels of IT, whereas anyone can register PayPal-communications.com
And as soon as the external agencies are involved, they want as much control as possible. I don’t blame them: setting up DNS entries for some stupid marketing site rarely rises to anyones priorities. So they route around internal controls. Everyone is happier, and everything gets slightly worse.
Edit: To clarify, there were a lot of good reasons not to allow them to link to a subdomain, especially they were running marketing software made by other companies, because there is a lot of nasty security issues around cookies with subdomains.
But there was no reason for them not to route their outbound email through a single domain, because the weren't supposed to run 3rd party mailers without deep audits.