This is one thing I've never been able to understand. Personally, I like to go mountain biking for the exercise of course but mainly to get away from the city and the noise. I see a walkman as a way of taking the noise with you. No thanks. I'd rather listen to the wind or birds or the sounds my tires/bike makes while I ride.

I think you're not fully "in the moment", you're not really taking it all in with some unrelated noise blaring in your ears. Plus other hikers/cyclists/cars/trucks/Tractors/747's find it extremely hard to get your attention!