Actually they do fail. If they aren't tight to the frame (and they never are when stock) they allow the arm to swing back and forth in the frame which causes bending and fatigue in the crappy sheet metal they use for the straps, typically at the arm bolt location. After a couple of weeks/months whatever the strap cracks then finally breaks at the fatigue point. I don't remember how many I broke before I went with the seat post clamp trick. A stock strap might hold up better if you wrap the frame like I said and managed to allow the strap to clamp firmly to the wrapping so that it doesn't allow the arm to move at all, that might help prevent/slow the fatigue point at the strap's bolt hole.