Not all of them sell real world copies specially of obscure indie games, Sseth is different, most successful "influencers" are attention whores that use the games as just a prop for themselves, people don't watch them for the games but for their "personality", most of them stick to AAA games (games that don't even need their reviews to sell anyway) because the objective isn't to talk about the game but the sweet youtube ad money and sell themselves as a "personality". Youtube is full of attention whores like that.
There are other "influencers" which content consist of making reviews on a process that wouldn't be strange on a sausage processing factory, they just place random video footage on the background and go rambling about the game on the most lazy way possible, their motto is the least amount of effort for the most bang possible. They go commenting about the features of the game on the most disconnected way possible, filled with barely developed subjective evaluations he quickly wrote down on some dirty napkin somewhere. You can see they played through the game but didn't even bother to actually think a little deeper about the game they just played.
Sseth doesn't do attention whoring nor produces content like an youtube sausage factory, he actually must waste a ton of time editing to make the gags to work as the gags aren't focused on him but on elements of the game so he shows the qualities of the game on a non autistic way that isn't a ultra lazy copy paste format that most youtubers do that manages to be funny with edgylord humor.