I've been thinking about this, too, lately.
I did pull all my works from AO3 and FFN. Everything since 2012. My accounts are empty now. While guilt was my primary motivator (I had a lot of wips and very few complete works outside of oneshots), I've been reflecting on how lack of community contributed. Not to say that I dipped out because of a lack of kudos or comments-- honestly, I probably got more than what was warranted-- but when I finally found a small circle of friends who would actively talk with me? Get as deep into the story as I was? Enable my unhinged narratives? Share songs or art that reminded them of the stories or characters? It was a whole new world.
The day I pulled everything, it was such a small, harmless comment that set it off. "Loved it! When's the next update?" The reader's first comment, 30k into the fic. It was almost out-of-body. What's the point? Why am I doing this? Why do I come into this space when I have friends somewhere else that actually talk to me? I get more out of reading bits of my fic out loud during Story Time on Discord with three others in chat than I do from any amount of three-word comments.
It ain't the numbers. I could have the fic with most views/kudos/comments ever and I'd still feel this way. It's interaction I want, and most existing fandom spaces really aren't designed for it. Views and kudos aren't interactions, but for some reason those are some of the most visible metrics. You interact with a specific fic, not the author. You reblog a piece of art and hope the artist doesn't read the tags too closely. Because many fandom spaces are oriented to the product, they're not oriented to the process or the people behind it.
Discord's not like that. Discord is oriented to the people in a given server, a vehicle for communication, not a destination in itself. Discord destroys the division between creator and audience because you are both and everyone else is both, even if the only thing anyone is creating is conversation. Discord's biggest limiting factor is fragmentation. Servers are generally purpose-specific, which inherently limits who you can talk with, but when given the choice of a large silent audience and a small interpersonal one, I think more people are choosing the second.
And it makes sense that people gravitate towards it, because AO3 and even tumblr don't really let the audience gush together about something they love. It's not common to see readers talking to each other in a comment section (I think I saw it maybe three times? in ten years?) and tumblr doesn't lend itself well to conversations (it happens but the interface is awkward af compared to just... live chatting in discord).
I agree with everything you say, 100%. It's lonely to be a creator. But it's lonely to be the audience, too. The best AO3 and Tumblr can offer right now is one-on-one interactions (which can be intimidating for both parties, let's be real), but once you do that... what's next? Is there a secret corner of tumblr I don't know about where people are having full-on conversations about fic or art? (completely plausible, I'm still fairly new to tumblr)
I don't have any solutions. I've found what works for me, for the most part. I'm working to be better about reblogs and comments, though I'm still kinda ass at both. I'm still getting coached on how to properly use tumblr (shout out to @bluedaddysgirl for having the patience to explain with small words). Like you say, we create for ourselves but we post for others. When others don't say anything, well... maybe someday I'll post on AO3 again. For now, I'm happy to share the gDoc links with those who ask.