Because it wasn’t possible for Switch 1. After some time, some alternative USB-C hubs supported the Switch video output but it was basically reverse engineering and I totally remember this first "compatible" hub back in the early days of the console which happened to brick consoles.
Nintendo is like Apple they :
- Don’t want you tu use the Switch in unpredictable ways : with the switch on the dock the cooling is guaranteed to be efficient, even on Switch 1 because it meant that the console wasn’t lying on a blanket.
- Don’t want you to buy anything else than their expensive dock.