That is only true with monastics and the Buddhist they have hair. It's not because you would want to save shampoo.
But we want to remind people that we are already monks and nuns and they should not try to make us their husband or their wife.
That's the second reason.
The message is very clear. And yet there are people who still continue to.
They approach monks and nuns in order to possess them, to make their own property.
So it's really helpful to shave your head in order to tell people that we are already monks and nuns. We want to live the life of a monk and nun. Celibacy.
We want to devote our time and energy to the practice and service.
And the third reason is that the Buddha said, "monks and nuns, every morning when you wake up, you should touch your head and you remember that you are a monk, you are a nun. And you have to practice truly as a monk or as nun.".
That is a reminder.