Why do monks practice shaving of the head?

As ascetics who shun material fanciness they do not value the fashion of hair styles.
And shaved heads don’t get lice and in all ways is seriously low maintenance.
To the extent they would have liked their hair, shaving it off is an exercise in taming the ego.

It’s an act of renunciation. In the culture of the Indian subcontinent of 2500 years ago, this was an outward demonstration of an inner resolve not to seek fulfillment in “affairs of the world”: career, family, possessions…

Having a shaved head made them publicly accountable. Monks were held to a higher standard of conduct and materially supported by the lay community. They’re holders of a public trust. With that shaved head, they couldn’t turn their monkhood on and off when it suited them.

It’s also a constant reminder to the monk or nun, because it has to be maintained. Every week or so, as the hair grows back, you have to renew that act of renunciation. It’s not a “once and done” thing, but an ongoing endeavor.

And finally, as @dabbler said, it removes a cause for self-concern. As a monk, you’re not tempted to devote much consideration to what look you want to create for yourself.

Not all monks shave their heads. Here is a Tibetan monk.

Monks such as Buddhist do shave their head open as part of their vows while Franciscan monks shave with narrow ring of short hair which symbolizes their renunciation of worldly goods. These practice are called “tonsure”, as a traditional practice of Christian churches done by monks, novices other religions such as Buddhist and some Hindu streams.

Interestingly, similar considerations apply to some Orthodox Jewish sects. Men do not cut or shave their hair (or at least the sideburns); by contrast, married women shave their head and wear wigs or cover their own hair with a wig or head scarf when outside the home.

There are nuances in the customs and rituals but there are a lot of fraught issues around hair.