Washer Dryer

In my entire life, growing up in my parents house and after leaving as an adult, I don't ever remember having the washer or dryer break. It probably happened sometimes when I was a kid and my parents just took care of it, but even if it did, the point is that it has happened rarely. I own a couple of rental properties and the washer and dryer in those break routinely, at least once a year. How can tenants break washers and dryers so often? Are they washing their dishes in them? Are they just throwing in a bunch of gravel and pizza boxes just to stick it to me? Gah!