Well, I don't know any experimental data, but my theory is that constant exposure to sound/music/noise even at low volumes is sufficient to damage the ears.
I did the rock band and the guitar thing for a while, but generally have been pretty protective of my ears.
I'm in my early forties and still have some hearing loss.
How do some some musicians avoid it?
Probably just luck, I guess.
EDITED to add, I haven't found hearing loss, or occasional tinnitus to be much of a problem in music-making. As long as you can hear something. IMHO, that would be a problem for people mixing and mastering recordings in the studio, but as a musician? Meh, I don't find it's a problem: you know what you're playing, and if you're in an ensemble, there are all kinds of options for getting a feed in your monitor(s). From all the way just hear it live from the stage and observe visually what's what, to using some custom in-ear monitors, through your own hardware mixer.