Because increasingly idols are Popctors (pop actors), their fame is due to their performance like with actors and not the content.
Nowadays the whole process is entirely assembly-line.
Composer - does the beat/music, there would be even a top-liner for the melody, then someone supplies the lyrics, someone else the choreo etc. basically like a movie.
But unlike movies, kpop still has an undeserved reputation of 'authenticity' that the idols are expressing their own experiences via art.
It used to be the case and there are still many who have that authenticity factor - who compose, write etc., G-Dragon, IU, Suga to name a few. But most others are only for the 'performance'. Popcting.
Sure they have singing, dancing skills, but that is rather peripheral to the presence.
But this is not limited only to Kpop, most commercial music industries have gone to this model. Even in the West it was the case of long until broken by Bob Dylan and the Beatles.
So now industries have a mix of Performers-idols and Artist-Performers, there are even people who only create and never ever perform.
Beethoven supposedly could not play any instrument. So a pure creator and not at all a performer. is the opposite of the idols.
Most of the audience do not care about the 'authenticity', so the model works, like with movies. The actors are not expected to write the story, script, screenplay, direct or manage the cameras. they just represent all the work done behind the scenes. But in that representation they can add value and humanity falls for the representation, than the substance.
Kpop is also like that.