In my opinion, its always been about what is easier to market towards a larger audience. for example you wouldn't catch Radio 1 blasting out Slipknot's 'Before I Forget', although it could be considered one of their tamer songs, the loudness/lyrics/music and even the image of the band wouldn't appeal to a widespread audience whereas, for example, Espresso by Sabrina Carpenter would because she tends to conform to societies values/the song sounds calmer/is almost relatable to listeners, while pop musicians will a lot of the time (not always..) sing about their latest break up or the classic "the one that got away", metal musicians and bands like System of a Down will sing about the problems and issues of the world (Aerials, Toxicity, B.Y.O.B). However, it should be taken into account that this is not always the case, pop stars can sing about the worlds corruption while metal singers are allowed to sing about their break up. Even radio stations like Kerrang! fall into this trap as they tend to play the more 'mainstream' side of metal nowadays and sticking to nu, thrash, and heavy metal rather than embracing the entire genre.
I'd also say its because of the almost negative imagery that people have towards metal artists compared to pop stars. For example, I (F17) was asking my friend (F17, huge Swiftie/pop enthusiast) if she would come to a Cannibal Corpse concert with me to which she responded that her mum would never let her because of the "community surrounding it" and when I asked what she meant she replied "you know like the drugs and stuff". In other words, to most people metal music is that one dodgy guy you see out in public that you cross the road to avoid walking past and pop music is the nice lady you stop and ask for directions from.
In the end I guess it all comes down to personal taste as well, while there are other factors (as mentioned above) that come into play people do just like what they like whether its black metal or pop. What seems like "just noise" to other people is what metalheads like, while to us pop music sounds, as you put it "tantalisingly monotonous" (while I am inclined to agree) is what other people like.