In all honesty, I see people listening to music everywhere I go. It's on their laptops, ipods, hi-fi systems, it's in elevators, in their cars, in the stores where they shop, they hear music of one kind or another while on their exercise equipment, while jogging, doing chores, when they goto church etc. I come into a dojo where Ray Charles is being played but soon, and well before another student shows up, the music is turned off. It is a white environment and it is silent--like a math lab. To my mind, the absence of music in dojos is remarkable that's all. I guess I might have asked, Why is this space, the dojo, so different from most other spaces--acoustically that is?
An interesting thing happened when I started to think about your question. As I pictured the situations in which people "listen" to music in daily life, the first one that came to mind was public transit: on the bus, on the subway, or waiting for same. Maybe it's because this is the situation where I'm most likely to have a pair of earbuds in myself. So the question is, what's the function of music in this situation? It could be a lot of things, but to be honest (especially for those of us who ride the Green Line), perhaps the most important is to drown out or escape from the present reality. This ride is terrible, we're packed like sardines, I'm tired, I don't want to be here. So you "listen" to something to take you away from all that. But "all that" includes the people around you, and while it may sometimes be beneficial (in a narrow sense) to ignore the people around you on the subway, when you're on the mat, the opposite is true.
My sensei often says, "You're trying to make two nervous systems work together." This is true in partner practice, and it would be true even in an adversarial situation, where you're being attacked. You can't just go on dancing to your own tune, oblivious to what's going on with the other person. And you definitely don't want some recorded third party telling you what the tune should be.