And if an Youtuber doesn't do livestreams, but only videos which aren't longer than 30 minutes, does it also take a lot of time ?
Consider for a moment that you're never seeing the entirety of what goes on behind the scenes. Particularly for Minecraft, a 30 minute video can frequently take hours if not days of building, designing, re-building, resource gathering, and testing redstone to complete. Even channels that do nothing but PvP or mini-games can spend ten times the video's length playing through loads of boring matches until they play one that's interesting enough that the footage will contribute to a watchable video.
Then there's the video editing. If someone puts thought into their editing, it can easily take four or five times longer than the video itself to edit all of the collected footage down and make it into something with a concise and cohesive narrative. After that is managing audio, selecting music tracks, making sure any dialogue matches up, re-watching the entire thing to make sure the game audio didn't de-sync, and so forth. For channels that focus a lot on presentation, this can be even longer.
On top of that there's all of the other things that go into being a successful Youtuber. Reviewing analytics, creating thumbnails (often multiple for each video since doing so boosts analytics), managing social media, making sure that Youtube hasn't done anything stupid with their account, and so on and so forth. Those channels are businesses that are often run entirely by a single person or a very small team of people. The later case also means going to production meetings and managing things like employee benefits, payroll, and so forth can also make demands on a Youtuber's time.
Most of the Youtubers that do it professionally work 12 hour days to bring you those little 30 minute videos. That's not a joke, that's the hard reality of working in a high-pressure video production environment. So when I say that they don't have time to be forum admins, I mean that they literally DO NOT have the time! Not if they want to eat and sleep anyway.