That's true, but it's much easier to pay for the 20% of a car that you actually use when you're renting it as needed.

While it's less obvious with housing the same applies: People buy houses that are bigger than they need at any given moment so that they have the space they need to entertain guests, or house prospective (or past) children, etc. It's much cheaper for me to put my parents up in a hotel when they come to town, than to keep a guest room idle for the next 20 years of my mortgage.