It's kind of just like, this sense of the "safety" of the walled garden (which one might argue the overlords could rugpull at any moment, which kind of happened with Twitter, but that's not something that really concerns me personally enough to trade it for the complexity of decentralization).
A lot of it is presentation. Instagram is the one that I use the most right now, and I feel like they really nailed this in the "everybody can make content" era. When I'm creating content for IG, I know exactly how it's going to look, there's editing and previewing tools, there's a confidence that people are going to receive it in the manner I intended and expected, and it's tied to the culture of that platform that I've become attuned to over time. The thought of a JSON of my IG post existing in a clearinghouse that encourages people to remix it into whatever, just kind of gives me the ick. Even if realistically it wouldn't really be abused, I don't feel compelled to choose that. If someone comes up with a new app that does some things better, I can choose to migrate to that, but likely I wouldn't want to just one-click import my IG history. It will be different in some way and I would want to make new content for it.
I guess I just kind of see these platforms as bespoke communities that people choose to be a part of, more than fungible data frontends. It's very personal, and...social. It's not the abstract world of data that programmers live in. My social graph in each app is unique and I don't really want it abstracted to one monolith that follows me everywhere. Each one is a new world with a fresh start.
Now, the IG post ecosystem is a lot more complex than it is with Twitter. With Twitter being at its core just text, it lends itself better to different views, but still, formatting matters and there are really a lot of rendering decisions to be made in a modern Twitter client. And again it just seems weird to me to have this oddly fragmented community with unavoidably complex interop semantics rather than everyone just being under the same roof. User opt-in aside, they will all develop their own features and quirks and they and the users will have to navigate how those translate or don't. Twitter clients are/were at least unified by the Twitter API. Regarding your initial question - it's more than just the profile page, do my comments show up in conversations? Likes and follower counts? Or are there weird gaps and discrepancies everywhere from users that haven't opted in? What if one fork requires mutual "connections" and one uses one-way followers? Each platform will make their own decisions. It just sounds like such a mess, and people will spend so much time disambiguating (oh no I'm on this one so I see it that way...).
There were some efforts to fork Reddit but it has survived as a great example of how lots of different people with different interests and opinions can occupy different parts of the same platform, even if lots of them complain about it because it's not their perfect vision of what it should be (see programming languages, and pretty much everything that reaches a certain userbase).