Why are shoes excepted?
Some pairs of shoes see much more wear than others, and unlike adults, when kids don't wear a pair for a while because the weather is more suitable for other things (sandals or wellies) they'll often hit the next size up by the time they can wear them. So sometimes shoes will hardly be worn at all.
And a pair of kids wellies must see at least twenty pairs of feet before retiring. I don't think these actually ever get to the stage that they are worn out: they just get lost somewhere on the way long before that.
If we get used shoes we
At least that's my experience and interpretation of my kids, but YMMV.