This feels like a basic misunderstanding of how the fediverse works. There are instances that embody your preferences and you can sign up for them.

One of the most important reasons I believe it is so useful to have a federverse that allows defederating is because ever since 2014 and 2015, and growing since then, there's been a phenomenon of rabid online trolling and hyperpoliticization that's had tendency to take over and destroy whatever pre-existing culture and norms existed, and the people doing it have leveraged bad faith free speech arguments to attempt to expose more platforms to their behavior, often making the same copy paste echo chamber argument that you are right now. I found the people making this argument to be operating from really shallow understandings of what intellectual diversity really means, because these people tend to ignore important components such as the paradox of tolerance, they tend not to believe that trolling or harassment campaigns are real, they tend not to be able to distinguish between "echo chamber" and the high level of discussion that's possible when you found a community based on a common interest or shared set on principles, tend not to understand that you're actually reducing the diversity of ideas by destroying each communities and turning all communities into the same thing, and tend to think of the full range of human ideas is represented in the unfortunately narrow framing of left-right spectrum which is most pertinent in American politics.

And for the fediverse, it calls the bluff perfectly, because for people who are concerned about echo chambers or "exposure to ideas" (yeah, which ones??), such people are able to join an instance that gives them the thing they say they want. But what they really tend to want is unmoderated unfiltered exposure to a captive audience, and the tangled contradictory mishmash of arguments about free speech and being open to ideas are just a means to that end. And so, they tend to be completely empty-handed when you ask them to explain why they feel specific instances need to federate or de-federate, you just get vague nothingburger speeches.

To be clear I don't think that everyone making the argument thinks that way, I think some people are unwittingly doing the work of bad actors without meaning to. It's just that I've seen this argument made over and over, and I feel like there's some sort of boot camp we should all put ourselves through that involves understanding the history and some core ideas, because it could save everyone a lot of time.