It is a problem, but I think it downplays the reason those platforms got popular.
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No admin required. No updating of software to make sure you're not going to get compromised by a vuln.
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No account management. You don't have to make a new account, and manage another password for every community you use. Also, no worrying about 1 when somebody like me can't be arsed to update that forum software. I don't want an account for everything.
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It's all in one place. You look at your "feed" of things and your stuff with a new post every week is right there with the stuff with new posts every ten minutes.
If you're running a big community you shouldn't be building it somebody else's garden, but you do need to manage the garden yourself and it's not super trivial and maybe your little Final Fantasy XIV group can make do with a corner of Discord and abandon it when it goes real shitty. If you've got 50,000 people, it gets a little trickier.
The Fediverse goes a little way to fixing things, but it's all a trade off. Not having corporations involved is a damn good start though.