Guitars are basically a wooden box with a big stick attached. Six metal wires are strung across this and tuned to exert, typically, 85kg of force – which just so happens to be the weight of the average guitarist! The wooden box and stick doesn’t like this and gradually folds up over time. Guitarists don’t like this and want it fixed. It would be desirable to adjust the big stick part but it’s impractical and costly, so a large number of guitars can’t be fixed.