Because the argument is (for most games) you're making the entertainment by playing the game. The developers makes the canvas and the tools, the YouTuber is the painter so to speak. It's not quite the same ripping clips from a film or football game that you had no part in making and talking over it
The hundreds and thousands of hours of hard work, vast number of skillsets and creative horsepower that go into crafting a gaming experience are not suddnely voided because a gobby twenty-something talks over it for thirty minutes.
But it's free advertising, some will argue, they're gettting exposure. True, they are, but I've yet to see a case where what the devs earned for their hundreds of hours of work is commensurate to what the Let's Player earned for recording and sharing their playthrough.
Markiplier (who seems like a decent enough guy) built his entire empire on Let's Plays of FNAF and Amnesia, but let's be honest here - who's done better out of that little arrangement?
YouTubers and streamers have been told for years that Let's Plays and streaming a game isn't fair-use, but they never listened because "Well, I heard it was.?
I don't even think general commentators like TB and Jim Sterling pass this litmus test. They'll normally sync up their commentary with vaguely related video content, but rarely is this stuff illustrative. Jim, for example, will talk for three or four minutes about Nintendo's business practises to a backdrop of say Splatoon in multiplayer mode, but really, there's no connection there. What does the way Splatoon plays really have to do with Nintendo's corporate attitude? Nothing, really.
The only ones that do fall under fair use, to my mind, are people like ACG and DF, where the content editied into the video is almost always illustrative of the commentary being made. They'll even freeze and zoom-in or run frame rate overlays, not to add a dynamic visual backdrop, but to illustrate their written point - like adding quotes to an essay.