I think it has to deal with what is most popular. And the most popular tend to be formulaic. Think of Gangnam Style.... that is REALLY formulaic.... and it became the most popular youtube song.
The 4 chords have been found to have those psychological affects on people listening to it.
Certain topics, like sex, love, or partying will always have a popular base. It is easy and it produces money. Consider it the music equivalent of C/B list, safe bets in making money. I can easily identify 5 or so people who've sung about sex or love in the past year, like that.
I don't remember where I heard this, maybe it was the radio, but Nicki Minaj(don't listen to her...) made a bet with another actress stating that she can make a popular song, even if she repeated the same words over and over throughout the song.... well she won that bet.