Why does (my) NUC/Laptop use odd (19V) power supply voltages?
Your assumption that if the supply was 20 V, it could be used directly for USB-C doesn't take into account the fact that the there is tolerance on the supply voltage.

That tolerance depends on what supply you're using. If you're using a 20 V supply that for some reason outputs 19.5 V then how could the USB-C output deliver 20 V? It would be unable to do so and would then be "out of spec."

To guarantee 20 V on the USB-C output, a guaranteed 20 V (or more) is needed, that means that it would typically need to be around 21 to 22 V to account for tolerances. That means 1 to 2 volt has to be "burned off" (using a linear regulator) which is inefficient.

A switched converter could be used as well but that generally makes little sense for small voltage differences. You'd really need a buck/boost converter to support a wide range of input voltages.

Since a buck/boost is needed anyway it can be more efficient to make 12 V from 19 V (easy using a buck converter and that would work over wide tolerances, if your 19 V supply was only 17 V that could still be enough) and then use a buck/boost converter to make 5 V, 9 V, 20 V for USB-C.