Why are the Wii nunchucks called nunchucks ?

because it isn’t a traditional joystick. a joystick is more associated with a pad with large buttons that requires A) both hands to operate, or B) a large arcade machine.
It is called a Nunchuck because it looks similar to the Old Japanese weapon of nunchucks. Two wooden or Metal handles connected together by a chain that is used in a swinging motion. Thus incorporating the Use and the visual representation.

The component called the “Wii nunchuck” is what gives you the facsimile of nunchaku.

If you could use the “Wii nunchuck” as a standalone piece, then yah, maybe it should be called the joystick. But you can’t. It must be tethered to the Wiimote in order to function at all.

And once you’ve tethered, you’ve got your “two weapons” attached by a “short chain or rope”... a.k.a. nunchucks!

Y’dig?

Because nunchucks are freaking awesome.