That's because their “landing pages” (project advertisement websites) aren't instances. The home page of any instance (e.g. https://diode.zone/) will show you the content visible from that instance.
But, if you’re going to be called “join peertube” you need to have the sort of content that makes people want to join: copy about what peertube is + a list of instances isn’t that.
I know it's not an instance and blah-blah-blah, but those are technical details that (possible) users don't give a single F.
I host a peertube instance that currently mostly contains my own videos. If I want someone to watch it, I link to the channel or the video in question. I don't say "watch me on peertube". The fact that peertube is the software that runs the service is not something a regular user should care about.
Peertube is federated using the same protocol as Mastodon, but federation isn't completely open by default. I guess this is for the better, because there is a lot of content you wouldn't want on your own instance's main feed (most instance hosts don't want porn in the middle of a feed of mostly tech videos).
Also, the UI of peertube isn't that great when it comes to following channels on other instances. If federation breaks (which it usually does) you get a nondescript error saying that something failed.
That is a technically correct answer, but it misses the point: they're asking users to sign up before they even see the product. They're expecting that the premise of the site alone is enough to entice users to join, which is very optimistic. It would be better to immediately direct the user to content, and then provide reasons for why they should get an account. At a high level, that's what YouTube does.
Just consider publishing more content on the homepage would raise questions, like what content to prioritize/ban from the homepage? There's some editorial work to delist certain instances from the instance list, but personally i'm rather glad Peertube doesn't go much further trying to recreate the whole Youtube experience where a single actor decides on what you get to see.
Yeah, like I mentioned, PeerTube is currently targeted at those attracted to the premise alone. Most people don't notice things like you did, and they're not going to interpret the apparent lack of features as a plus. If it's going to gain mass-adoption, it really needs to work on first impressions.
Some of these suggested videos may be fun to watch but Youtube has become really bad over the years to discover interesting, not well-known content. Now they're trying to maximize ad revenue, and the recommended videos have suffered as a consequence, only pushing top-popularity content pushed by "content creation industry" with very little quality.
...so they're not interesting to you. I see several news and music-related videos that I might watch. Like I said, YouTube presents you with a set of videos you may be interested in, and even if you're not, it also gives you tools for finding videos yourself (e.g., through tags & categories). The moment you start watching videos, it starts refining recommendations and funneling you towards more content.
By contrast, PeerTube's homepage is pretty bare. I don't even know what the interface would look like if I did sign up.