A lot of folks with those nasty infestations that have burned images into your brain also have too many birds for the space they are provided and the birds cannot take proper care of themselves for lack of good dust bathing options. And the keeper is typically lax on proper husbandry. Nuf said.
A point that I don't know if I've ever seen mentioned before. It's got to be easier to deal with an infestation if it occurs when you've got plenty of space to work with.