Do popular artists write their own music?

Okay i was originally a person who only listened to rock genres, but then i started listening to the radio and well all there really is on is r&b ie. kesha, bruno mars and such. Its alright i like it but i wonder do they write their own music i just see them more as figures than actual musicians and with all the partying they probably do when do they think of their music...i dont know i just see them as people given the spot of a celebrity and they just ride it.