It's really about how much a false positive affects your IP's reputation.
When you are sending out a lot of email, recipients see a volume of mail. If a few of these get marked as spam, no big deal, you sent out 5,000 mails in an hour, of which 2-3 were marked as a false positive.
If you are running your own VPS you are sending out comparatively fewer mails so the decision on whether you are a spammer or not is made with a lot less information -- and most recipients will assume you are a spammer immediately.
When you are sending out a lot of email, recipients see a volume of mail. If a few of these get marked as spam, no big deal, you sent out 5,000 mails in an hour, of which 2-3 were marked as a false positive.
If you are running your own VPS you are sending out comparatively fewer mails so the decision on whether you are a spammer or not is made with a lot less information -- and most recipients will assume you are a spammer immediately.