Why? because the unit price is lower? Can't zillow just... buy less houses? I don't see anything different between zillow buying a $500k house and one hundred used cars worth $5000 each.
Easier to sell 50 cars than 1/2 of a house tho.
While I don't doubt this effect exists, I seriously doubt it's a significant factor when you're dealing with 7000 houses.
Also, every car has scrap value. Houses don't have that.
The equivalent for houses is the land they sit on. In metros that could easily be worth more than the house, and in most cases it's worth more (in % terms) than the scrap value of a car.