Katy Perry found success as a songwriter before she found success as a singer, and has written songs for Britney Spears, Kelly Clarkson, Selena Gomez, and Nikki Minaj, among others. (https://www.rollingstone.com/music/music-lists/katy-perry-wr...) Most of her own songs are co-written in studio with her collaborators on the song (meaning the feat artist and the producers), with each collaborator pitching in or more of the hooks or verses.
Ariana Grande has written a few of her own songs (though none of these were released as singles).
Perhaps you meant that neither Katy Perry or Ariana Grande are composers (who write the instrumental portion of the song)?
I think they mean "Katy Perry" and "Ariana Grande" are fictional characters as the public knows them. They are characters played by two artists and supported by media/storytelling.
This reminds me a bit of what I've heard about pro wrestling. There's a bit more story and theater to pro wrestling, but a lot of times the characters' personality derives from the person 'playing' them, with the knob turned to 12.
For others wondering about this style of entertainment, it's called 'kayfabe'
"Kayfabe, in the United States, is often seen as the suspension of disbelief that is used to create the non-wrestling aspects of promotions, such as feuds, angles, and gimmicks in a manner similar to other forms of fictional entertainment. In relative terms, a wrestler breaking kayfabe during a show would be likened to an actor breaking character on-camera."
Outside of the entertainment industry, politics is an obvious example, though wikipedia states:
"It has long been claimed that kayfabe has been used in American politics, especially in election campaigns, Congress, and the White House, but no evidence of actual usage of kayfabe in Washington has ever been uncovered. In interviews as Governor of Minnesota, former wrestler Jesse Ventura often likened Washington to wrestling when he said that politicians "pretend to hate each other in public, then go out to dinner together.""
I find the idea of being fed a story fascinating because a successful story feels more real than life itself and I'm not sure you can separate the two (life and the stories we tell ourselves). Once you can see it for what it is, that it's just a story, it breaks the illusion. But a really powerful story sucks you back in time and time again, and sometimes, you forget that you were being told a story in the first place.
And now after typing the above paragraph I sit wondering how many stories I currently believe are real, are just stories after all. Money being one of the most powerful.
Ariana Grande has written a few of her own songs (though none of these were released as singles).
Perhaps you meant that neither Katy Perry or Ariana Grande are composers (who write the instrumental portion of the song)?