Why? The same reason rotten meat and piles of poo produce maggots. The meat and poo don't actually produce the maggots any more than your belly button produces lint. It's just a friendly environment for flies, maggots and lint to collect. It's common for some belly hair to grow towards the belly button, which makes your belly button function like a lint magnet swallowing up lint delivered by way of the hair covered belly which, when shook like a bowl full of jelly hastens the delivery of lint to the button.