How long have you been part of the fediverse? (A term which tends to not be capitalized, by the way. *nerd snort*) It's not about you getting to interact with every instance using just one account. It's about putting the power into the hands of ordinary people. Including the power to associate or disassociate with certain people, communities, and content. That includes an admin's ability to go "I see you're not sufficiently moderating your instance. We will defederate until you've taken steps to ensure your instance sufficiently moderates with common-sense rules.". Whether that is due to some content policies or to block an instance from which a ton of spam originates.

Just how with email a provider can choose to block or automatically mark-as-spam any email coming from a server they don't trust, for example because it's a known source of spam. It's actually how a lot of the internet works. And it works as long as well-intentioned people are in positions to make such decisions. And if a server or service goes rogue, they get the equivalent of defederated.