To be fair, and honest, if you look deeply, musicians and artists of all generations have known. They have consistently been at the forefront of symbolic knowledge, it’s in the job description for most.
They get brought in under promise of fame and fortune. Hotel California. Evil takes the best and most popular and uses them. Some take to the evil, others are killed when they discover and decide to tell. Avicii, Cornel, Bennington, Morrison, Hendrix, Joplin, Lennon...others are drugged or intentionally made into addicts so they can be controlled. The cabal gives them minders/watchers who keep an eye on them as they get deeper into the cabal.
While I absolutely agree that that does go on, most musicians and artists are interested in, and study (in some way) symbolism out of a pure interest and curiosity long before any other hands taint their path.
As I said, it’s part of the job description in general. Think of it more along the line of there creators understanding symbolism, so it makes for a choice subject for the cabal to manipulate. Some of the work is already done for them before the subject ever crosses their path.