Indeed, gaming is extremely prevalent on YouTube. It's basically what made YouTube popular. It's easy to be a gaming YouTuber, it doesn't take a lot of skill beyond being able to play a game or talk about it. You need only to record gameplay of some sort (playing the game, making tutorials, recording weird glitches and problems and even reviewing games) with or without a camera or commentary, edit it with intros, outros, music and animations and upload. It's really not that hard to create. The hard part is making content that people want to watch, that they will enjoy. There may be a lot o gaming YouTubers but most are not really successful.
Cooking however requires some skill in cooking and commentary. Just as video/photo editing tutorials do. Channels that involve fishing are usually based on a passion for the activity that not a lot of people have or are willing to make videos about. Then there's the music genre where the person is usually very skilled at making music and/or singing. A skill not as common as we thing.
In essence, unlike most other genres, being a gamer doesn't really require having skills beyond knowing how to use a PC and edit. Even if you're the best at, say, Fortnite, Sea of Thieves, Minecraft or even Uno, more often then not people who are just average skilled who happen to be funny are just as likely to be popular. It's simply easy to be a YouTuber gamer. It's just not easy to be a popular one. That's why there are so many.
I'm one of them. LOL