Hi!
For me, the question can be seen in 2 different aspects:
Why do other people do that?
First of all, everybody played, plays or will play video games. Before you would say no, think about it, think about the girl who plays on his iPhone, think about who invites you to play FarmVille, and now, you did get that everybody plays video games, so that actually means that the whole theme, Gaming has a really big possible audience, so if an average person, wants to try YouTube-ing, there is a big chance that he will do gaming, because great possible audience means => big audience for me, but that's not how you slice up the cake, so really little amount of people get the biggest slices, and thousands or hundred thousands people will get the smallest slices.
And as said upwards, this is the easiest to get into right? For an average person of course, they think you just sit down, press a button, play a game, say something while, press a button and upload it (and also ther are no special requirements, like you don't need a 2000$ equipment for it). Okay, so that's the reason probably, why there are so many gaming content on YouTube, but that is also the reason why it's damn hard to show something unique, and that is why it is unlikely to get big, or get profitable from gaming, and since it is "easy to do" there are a lot of "easy to do" videos on YouTube, since there some popular ones, which are popular because the reasons somebody said earlier. Because of that there many low-quality gaming-related video on YouTube, and the same reason, why it is hard to do it, but still because everybody plays games (okay playing games =/= gaming, but don't get really into that), it's still the most popular content type on YouTube and that is also why gaming-related videos are mainly underrated since there are lots of low-quality videos, and one thing left here, that who ARE popular from gaming, can be also popular, because as I said, everybody plays games, and since everybody plays them, they get into the same parts of games, and people just like to see how others react to that part of the game, becuase humans are curious.
And, why I personally do that?
For me, gaming is mostly part of my life since I was 5 or so, and I find the innovation in that part of IT, I can see the artistic part of gaming, since I think, as I said, games can be art. Just look at the graphics, the sound, the story, just like probably what were movies in their time, is games now, but I don't want to really get into this, so continuing the main part, it was part of my life, I just love to do it, and as everybody, I have my own opinion, and that is an opininon that I want to share with the world, because there are a lot of people who actually care about gaming (said in upper parts), and also there is always something in gaming that can show something new. These are introduced mainly by indie games, and some get their shine, but many don't, though they have their unique innovations, but they miss their shine, and I want to well "help" these games, by showing them to the gaming community, by my videos, and that is mainly why I do gaming related content, and also gaming is fun, and who doesn't like doing things what are fun? Sharing fun things is just a cherry on the top.
Thanks for reading!