Right but a truck could be full of not 100 packages for 100 houses, but 100 packages for 70 houses. Both are full, but one will require fewer miles driven, hence be more (fuel/environmental impact) efficient (not necessarily time efficient).
They presort regionally and the trucks at the delivery stations are loaded with assigned containers/pouches of packages. It doesn’t make sense to hold a truck for a pouch and doesn’t really save anything to have ground covered twice.
They do other stuff too. They schlep heavy stuff on UPS, and hazardous or liquids usually go USPS.
That isn't really true, though, because these houses exist in the real world and aren't just buckets to be visited.
Your conclusion would only be true if houses are all equidistant from the warehouse. That truck with the 100 packages for 70 houses is going to have to drive by more than just 70 houses to complete its route. It would not add additional fuel or miles to stop at those houses while it goes.