How come so many rockers are hard of hearing?

Is this a question . That needs my medical expertise to answer? Seriously? Tell you what, do your own study. Go on tour with a band for as many years as they do. Let us know how's your hearing. Make sure you sit in the front row though.
Multiple factors. Most work hard in factories with noisy rquipments, some could be medications the notorious one is beta blockers which helps heart but hurts the ears.