They can't prove a negative, but they valiantly try, often using the very points that are excellent reasons to self-host as reasons we should all just give up.
"It doesn't work for me, so it won't work for you" is rather weak when the reasons aren't articulated, when generalizations are made without detail, when hardly any attempt at all was made to find the root of the problem. Your failure is not the same as my quarter century of success.
They also tries to make everything all or nothing, which plainly lacks any imagination. I wouldn't want an admin on my staff who tries something, then gives up at the littlest of hurdles.
There are plenty of issues, but there are also plenty of solutions. You're having issues sending because your netblock sucks and you don't have money for something better? Pay a few dollars to smarthost, for instance.
With Google and Outlook, you get no determinism, no accessible logging, no clear view about their filtering rules, no assurance that your outgoing email won't get blocked, anyway, because of the tremendous amounts of spam these monopolies allow...
We should encourage MORE people to self-host email, not less. We should never just assume the monopolies are the best we can do.