We tap our helmets to warn of cops. We wave (or nod in countries that drive on the left) just to say hi. Absence of a signal would be useless on a bike as we'

We tap our helmets to warn of cops. We wave (or nod in countries that drive on the left) just to say hi. Absence of a signal would be useless on a bike as we're generally pretty bad about giving the signals we intend to give, let alone not giving the ones we don't intend to. It would be chaos.