Why are black and white cats called tuxedo cats?

because the fur is usually black and white with the white on the belly as if the black is the tuxedo jacket and the white the under shirt

To be clear: Only those with that specific pattern (mostly black, white on the belly, neck, and possibly paws and muzzle) are "tuxedo cats." Cats where the black and white portions are distributed more randomly are not tuxedo cats.