I'd agree with that on the surface but those platforms you listed are basically forums with a new name! They provide very little over and above that. So, in actual fact, they're only made more complex as a result of rules added by the owners to squeeze as much data from every visitor as is legally (and illegally in some cases) allowed! So they provide nothing of value above a regular forum imo.
In reality, they are lists of submissions from end users that have been categorized, however the categorization engine happens to be algorithmically generated, rather than ordered by date. And the companies themselves are doing whatever it takes to monetize your eyeballs regardless of whether it affects your health or not! They are abusing every visitor to the maximum extent that the laws allow!
Their business model is to centralize all forums on their platform and for that there is a massive cost at that scale. That's their choice to host all forums! However to pay for that they are abusing every visitor.
Using Reddit, a single forum about Ubuntu (/r/Ubuntu) would cost around $5 a month on DO for someone to host themselves. Using something like BBForum or something similar would not require much in the way of maintenance.
The point I am making is that all this stuff existed long before the big tech platforms. It can easily exist without them too. They are not actually providing anything of any worth that couldn't easily be found elsewhere (well, used to be found elsewhere!).