I remember being bored in school and picking out the 'string' that had formed out of grease and dirt around the x/y axis rollers, I don't think there was anyone whose actual job it was to clean those things
I have it with the new Laser Mouse at school where the grease is formed at the corners of the pad. I can always feel how icky and sticky those mice feel like so I actually need to remove them somehow.
It's just different levels of sweating / releasing fat. My mice and guitars are always more grimy than a regular person's. I try to keep my hands clean, it's just my body works a bit differently