Q. Why are there no Jewish monks and nuns?
A. Judaism has had its ascetics, but normative Jewish teaching does not approve of withdrawal from the world.
JH Hertz points out in his Torah commentary that the famous chapter 19 of Vayikra presents holiness not as a flight from reality but a programme of holiness within reality – not being holy by abandoning the world but engaging with the world and living a holy life in the midst of marriage, business, agriculture, etc.
The monastic life seems to view the world as irredeemable; the Jewish thinkers speak of the holy and the not-yet-holy.