You'd be surprised how much some people make.
It seems like the top earners for some reason are people who stream themselves playing video games... apparently there's a huge market for that. Then there's the prank channels, which require very little effort and make lots of money. Then there's the vlogs of people who are usually already rich, and you get to watch them riding around in their lamborghini and going to the gym and whatnot while filming themselves and trying to pretend to be inspirational.
Food channels are another huge earner. Here's the (very unofficial) estimates on a few channels I watch:
VahChef: $2k-24k per month... $300,000 a year on the high end just on ads. He also gets paid to use certain kitchen items and cooking utencils in his videos, and had a series sponsored by Philips airfryer/blenders
https://socialblade.com/youtube/user/vahchef
Mark Weins : Estimated $540 per day, so $200,000 per year on youtube ads. He also runs a very profitable blog on top of his youtube channel. His popularity also leads to tourism agency's in different countries/locations paying for his travel to shoot episodes in their location, so he travels and eats for free very often.
http://naibuzz.com/2017/03/26/much-money-mark-wiens-makes-youtube-net-worth/
The Food Ranger: estimated $400 per day, $150,00 per year on youtube ads. Also is recruited by tourism agency's and comped travel and food often.
These are just a few that I like, and certainly not even close to the most popular on youtube. Basically everything they do is something anyone with a camera could do, they just put the time and dedication into doing it.
I've considered making a channel just for spicy food recipe's for awhile now, but never have the time. Not sure how large the audience would be either, but it'd be fun since I love cooking and sharing recipe's anyway.