Just like DDoS attacks are forcing websites to hide behind a small number of major services like Cloudflare, spam is forcing email users to consolidate into a small number of well-known platforms like Gmail and Outlook. It's just too much hassle otherwise.

But unlike in the case of a website using Cloudflare's DDoS protection to fend off attacks, someone who gives up running their own mail server and signs up for Gmail isn't doing so in order to get any direct benefit from Gmail's spam filter. (Open-source spam filters have gotten good enough that they often do just as good a job as Gmail, if not better.) It's almost entirely because of the insane hassle of dealing with third-party blacklists as you said. Google and Microsoft should give them a medal or something.

Many of the blacklist operators are highly moralistic, too. You'd think they were on some sort of holy crusade against the ultimate evil. Of course you are partly at fault for e.g. not enforcing strict password policies on your users, but a small percentage of users are going to get hacked no matter what. I wish the blacklists would redirect at least 10% of their moral outrage at the danger of centralization that they're so willingly facilitating.