Theft is still profitable. They are maybe getting less money by selling for parts to a fence than if selling a laptop that can be resold straight away, maybe by themselves. Still some money.
Otherwise the thief could just steal the phone, wire in a hacked Touch ID that accepts any thumbprint and resell the phone.
This way it's way too expensive to be profitable unless you're doing it on an industrial scale. Most thieves resort to trying to con the previous owner into giving their Apple ID for unlocking.