It is 100% envy and jealousy. They make enough money to support multiple people (I don't know how much Josh and Jack make, but I think Jack does it as a side gig) and basically a studio (Their warehouse in Milwakee) with expensive cameras, lighitng and great editing. Compared that to their reviews which look like they are shot in a broom closet with a dorm light from Walmart. It doubly burns them because they honestly they don't give a fuck about outside criticism or social pressures and say what they mean. Which is so popular. They seem legitimately genuine. I think it only got worse with the 'nerd crew' shit where they openly mock others for being obvious shills and pathetic.
RLM also has a big thing of what makes comedy great: Self-Depreciation. They can laugh at themselves openly. They don't have a stick so far up their ass they immediately start screaming and crying. And if you are slef-depreciating already, there's not much anyone can do to you. That's how Plinkett came about. They thought it was too autistic to go point by point seriously, so they made this crazy insane murderer character do it. That's a huge point of the joke with him. That only a schizo nutbar would pick apart something as shallow as pop culture like that. Don't get me wrong, the criticisms are good and genuine, but they're caked in this self-depreciating, obsessive character to make it more light hearted. I still think they are the sharpest critics out there and go over what I care about (story, characters, plot, abstractness) and you get a good diversity (FROM STRAIGHT WHITE MALES) of opinion, with Mike being Shlocky, Rich liking good stories, Josh being eccentric, Jay liking weird, artsy stuff with Josh. So not only do you get a breadth of opinion, they're consistently funny and well written.
Compare this to other critic websites which just parrot each other, are heavily paid shills or just follow a singular party line. Its boring, its mundane and it doesn't attract and audience. I don't give a fuck if a movie is sexist or racist, I care if it is good.