They really aren't. But with Peertube and video platforms, it's a bit different than for example on Mastodon or here. You wouldn't want wo watch a Mastodon's ALL feed. It's just 400 random posts in a row and then one that sparks your interest. And no one wants to scroll through just loads of random, non-interesting stuff.
On Peertube the "All" feed - the one on the main page - needs to be somewhat useful. To deliver quality content and provide you with new videos to watch. Not just some random ramblings mixed with 3h Minecraft videos mixed with political videos that got banned on YouTube mixed with MLP...
And that's why many Peertube admins don't just include all the content out there. It mainly leads to people being annoyed and saying YouTube shows videos they're interested in and Peertube is just random junk.
But they aren't defederated. You can usually access that content, subscribe to those channels and watch all the stuff. It's just not displayed by default.
I think both approaches are useful. Curated content is nice. You can have for example instances for tech-related stuff, some for tinkerers and makers, some for gamers and some for poliyical drama. They form little bubbles and their peer groups gets relevant videos displayed to them. And they're not closed off. You can still subscribe to all the other content you're interested in.
(There are however also instances that subscribe to everything. And there is actual defederation happening. I don't think the Peertube software shows which instances are defederated to the viewers. There are like 4 instances with porn you might block as an admin and several dubious ones with fascist/nazi content and several more ones dedicated to fake news and covid misinformation. These are blocked on many instances, that is true. The drama behind that is easy to explain: Most people don't like fascists and don't want to support them or give them an audience. And the covid-liers came to Peertube after they got banned on YouTube and Facebook and it annoyed people. It was the same 50 videos with misinformation over and over again and shaky videos of people discovering "nano-bots" in the masks (if you ask me, looks eerily similar to black lint and dirt) and after is started to drown out better quality content and got annoying, we gave them the boot and went on with our lives. I don't think it's too much drama. And it's not that many 'bad' instances. I can't say how many instances other admins blocked but back when I tried Peertube I blocked like 40 instances and there was no good content on them anyways. And I don't think you're talking about those in the first place since it's hard to figure out which instances are completely blocked.)