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It's a combination of factors. First and foremost there are corrosive chemicals in your dishwasher. They're not going to hurt most of your dishes, but the edge on a blade is surprisingly delicate (well, most blades anyway). Unless you've got a VERY good knife it's not going to be able to stand up to that. Second, people tend to throw all their silverware, knives included, in one basket. The rub up against each other as the dishwasher runs. Now considering that a glass cutting board can do some serious damage to your blade, imagine what a steel fork will do. Third, the blades don't get dried as quickly in the dishwasher as when they get hand washed. Water isn't good for steel. Even stainless steel can only take so much sitting around in water before it starts to feel the effect.