most likely it's because nobody ever cleaned the sliders on the caliper. that's the number 1 cause for premature brake replacement at work. people just drive and never clean their brakes.
when the sliders start sticking, they won't allow even pressure to be applied to both sides, and you'll start burning up which ever side it's sticking on.
brakes should be taken apart a few times a year and completely cleaned. that way they'll last longer and work better.
if you can't move your sliders by hand, or they are insanely hard to move, it most likely already is to late. At that point just replace the calipers and replace the pads. make sure you lube the inside of the slider boots with high quality caliper grease before you insert the sliders. and whenever you clean your brakes, make sure you pull the sliders out of the boot and clean/re-grease them.
