When you post on reddit, instagram etc. you're giving the company your valuable data. (And it often is valuable, you put your thoughts into it and all that). Consider if you take 15 minutes to create value content, you'd get paid $5 or sth if you were doing it commercially. Now all that value is donated to big tech companies. (that's the only reason why they have value at all.)
So what's the problem with that? The problem is that reddit and others can instantaneously put your years-long effort behind a paywall. I'm not joking. Look at these:
when i want to see content on reddit (for research) or instagram (for local community) they don't allow me to see the data that users have contributed. Reddit claims that it blocks VPNs for "security reasons", meanwhile i'm using VPNs in browser's incognito mode to not be tracked. Instagram wants you to sign in sothat they amass more users and can take leverage of the network effect even more.
I can understand restricting who can post to limit bot activity, but reading must always be possible. After all, it's our data. That's exactly what happens when you give your data to big companies to "communicate with other people": your data becomes a source of income for them, at our expense.
Early iterations of the community did try to incorporate these features.
But simply tapping the sign that reads "Go read the wiki on this solved issue" doesn't generate the kind of traffic and community participation that repeating the same copypasta every month or two does. Also, god forbid one of your mods does something like...
post something unPC in a Politics community
argue with content from a native advertiser
piss off an admin... somehow
sign in from a device in a way that triggers an automated unchallengeable ban
fail to constantly scour your community of shitposts
Plenty of the old subs have been hollowed out, transferred, or otherwise denuded of their professional contributors for some stupid reason or another. And now the site is so aggressively policed that you can't really join (much less re-join once you've been booted) to add anything useful.
This has been the OG Wikipedia's fate as well, with mods getting older and links going stale and new contributors all getting hyper-policed for fear of bots, marketing accounts, and shitposters.
Small forums full of active users are more reliable for useful information than big forums full of lurkers and attention whores. That's not unique to Reddit. Just very obvious, given the long and deplorable history of the platform.