An addendum to Yaish's comments it's heat and agitation that shrinks wool.
The hairs of wool, and all hair for that matter,have scales that overlap. Think scales on a fish. When you warm the hairs the scales relax and open. If while relaxed you now agitate the hairs the scales slide over each other and make the hairs shorter and fatter. This is known as felting. If you cool the wool the scales lock down and stay there.
Never, ever put wool in the dryer! Always wash in cold water. You don't have to hand wash but that is the best method to clean all natural fibers. A gentle massaging is all the agitation you need.
By heating the wool back up you can recover some of the original shape, but never all of it. If shrunk to the point of felting they are there for ever more. Period. Done. Game over.

Sock stretchers simply hold the hose in shape while they air dry. I make mine from a thick wire. About twice as thick as a coat hanger. I spot heat the wire with a propane torch and bend it over a piece of pipe held in a vise. Never thought about offering them for sale. Guess I should.