And this kind of gets to the nub of it. Centralization brings massive economies of scale, and allows you to free ride off the deliverability of other newsletters that don't come anywhere near talking about potentially fraudulent activity. But you have to play by their rules and you're at the mercy of one day being no longer welcome, even if you've paid them thousands of dollars before.
Of course, you're welcome to go off and run your own server / roll your own email protocol / used a 'decentralized' platform or Gab or Voat or whatever, but often a) these things end up being a lot less decentralized than you think, and b) good luck getting anyone to know, care about, or trust you.