Same reason as they donate chipped or stuck together china, ancient, damaged, greasy utensils, broken toys etc. I’m a volunteer, I don’t sort and price clothing, paid staff do it, and you wouldn’t want to put your hand in some bags! At least dirty and damaged clothing can be resold as rag, and unsaleable books are also sold by weight. We pay for collections of other unsaleable items that really should have been binned not donated.
Sometimes much of my shift is spent washing glass and china, because its too dirty to display. We do put some expensive items on sale in the shop, if no interest they get sent to ebay. I buy very little btw, the occasional rag item, or a jigsaw. We get 25% off marked prices, or make a small donation for a rag item.
Most charity shops are short of volunteers, all the ones round here have notices in the windows. If anyone has time on their hands, give it a go!