Because they are trapped there. A user account on Reddit remains on Reddit, it can't access communities outside of Reddit.
Creating a new account on kbin here was not exactly hard. Is your argument that millions of people still use Reddit because they can't type in a couple of data fields?
Then when Reddit goes bad and startrek@reddit.com starts sucking, I can just start posting on startrek@startrek.website instead. No need to create a new account or "migrate" anywhere. The friction is minimal.
Right, but then all of the other users that post interesting content that you went to startrek@reddit.com for are still on startrek@reddit.com, not on your new instance. Now, your new instance gets zero posts because it's new, but the old instance still has millions of people posting to it every second of every day. Yeah, you have a new place to post to, but all of the content that you went to startrek@reddit.com for in the first place is still over there. Federation did nothing to help that problem.
You don't need to move to a different instance. I'm not sure where this miscommunication is coming from. You can continue using timbervale@kbin.social if startrek@startrek.website "goes bad" and instead go hang out on some other startrek community without having to create a new account.
It's not miscommunication, it's just that I'm removing the option of changing to a different community/magazine on the same instance. If I can no longer stand being part of !startrek@startrek.website, I'm not going to start posting to !startrek2@startrek.website, I'm going to the next largest community, which is, at this time, usually on a different instance all together, like !startrek@lemmy.world. I'm not talking about my user account, I'm talking about the community/magazine itself. If a mod on !startrek@startrek.website goes crazy and starts banning people for talking about Star Trek: Discovery, I'm not going to want to be there, even if my account hasn't been banned, yet. As a result, I would need to find a new Star Trek community to post in, which is what I mean when I say I'd have to move to a different instance (because why would I switch to a different community/magazine on the same instance? And, also, there are a million scenarios where switching to a different community/magazine on the same instance would be a bad idea/impossible). Note that when I say community, I mean the equivalent of kbin's magazine, as Lemmy calls it a community.
Move to the smaller instance. Everyone else can move too. It's just as easy for them as for you. Then it becomes the bigger instance.
The entire reason I would subscribe to a community/magazine is because I enjoy interacting with the community there, and seeing/interacting with the content they post. If I switch to a smaller community/magazine, that content becomes exceedingly rare, and the number of people I can converse with drops dramatically. There are 315 subscribers to !electricvehicles@kbin.social, but 241,000 subscribers to https://reddit.com/r/electricvehicles. Clearly, the experience of posting to one vs. the other is drastically different, wouldn't you say? Why would I go to a place where I have just a few people to talk with, when I could stay on the old site that has thousands upon thousands of people? The same applies to if it were 5 years from now and !electricvehicles@kbin.social has 241,000 users but !electricvehicles@lemmy.world has 315: no one will want to switch if the mods of the larger community/magazine turn into assholes.
If it's "bad enough" for you to move but not for them to move, perhaps you're being more sensitive to the badness than everyone else is. Maybe it's not so bad. If it is that bad, then why aren't they moving?
So now it's, "if most people don't move, it's not really that bad, or it's your fault for thinking it's bad"?
The benefit would be the content. Imagine you post to a magazine with 300,000 users, with new posts every 5 minutes, and hundreds of comments per article; that kind of an experience would be more desirable than posting to a magazine with 500 people, with new posts every 12 hours, and maaaybe 10 comments on the more popular posts, wouldn't it?
The benefit of every server moderating everything that comes in would be that a post that isn't suitable for one instance could be perfectly fine for another. Imagine the topic of politics: for some people, discussing abortion might be too sensitive, but others might be totally fine with allowing it. We wouldn't want to stifle conversations about that subject, though, so maybe it gets through to the individual instances to handle it as they see fit. This way a user can continue interacting with a large community that's interested in politics, instead of fragmenting that community into half a dozen smaller communities; sure, some posts might be hidden by some instances, and those threads would be less active than thread about more agreeable subjects, but that's still a lot better than every thread being less active, isn't it?