A common fallacy in business.
> Go full time crushing things with an hydraulic press? Yes! Go full time discussing medieval weapons? Yes! Go full time building things with no tools from scratch in the woods? Yes! Go full time trying to beat Super Mario as fast as possible? Double yes! And so on.
They're not that much a weird niche really. Go to any large university and there's probably a club full of people who discuss medieval weapons full time. Actually there's probably at least two (one that's more purist in terms of historical accuracy, and another that's more focused on fun), then there's probably also a fencing club, a kendo club, maybe a HEMA club, dozens of gaming groups.
And it's not like it's not niche that isn't served by TV. There's plenty of documentary / infotainment series on medieval weapons and combat. There's serious series. There's Deadliest Warrior. Youtube often gets viewers because it's a different format, not because it's serving a very weird niche. Cheap, regular content (and often the comment box keeps creators a bit more honest), suggested next episode, more talky, short episodes.
As for those other examples, there's Mythbusters (smash stuff!), survival shows (primitive technology in a different format). Gaming is the only really odd one - it just isn't shown on TV much.
Go full time crushing things with an hydraulic press? Yes! Go full time discussing medieval weapons? Yes! Go full time building things with no tools from scratch in the woods? Yes! Go full time trying to beat Super Mario as fast as possible? Double yes! And so on.
My point is, this revenue is more than enough for interesting content to be produced, content that would never exist without this kind of monetization scheme. It might not be the best model for high end documentaries or movies, but it works for a different set of content which is still highly sought after.