It has everything to do with your hand getting dirty. It is a common fact that string companies recommend you change strings on a guitar about once a week. At first you'd think they're just trying to make money but in all reality it is a good habit to do if your playing alot. Not just live but even more crucial in recording. This gunk per say you find under your strings also dampens your tone and is one of the main reason your strings get dull after awhile. Now if our hands were perfect and clean at all times and never produced any type of oil then I can see where it is no problem to leave your stings alone, they would last a lot longer but we produce alot of oil that it can damage our strings. There are a few products out there to clean strings so you dont have to replace then as mucha sn also look at the nanoweb ones they last longer because of a coating that helps them not collect that gunk as much.