It doesn't work that way. Once a car is obtained, it has fixed costs. My monthly finance payments can't be pro-rated to the days I actually use the car. Ditto for insurance. In many cases, the best deal for insurance is to pay for the year up-front. Bears repeating, a year, up-front. Insurance companies incentivize this. Between those two, that's 80% of the TCO. Fuel and maintenance are actually incidentals based on usage and account for < 20% of annual ownership costs.

Ergo, it's no surprise that people want to use them as often as possible. They want to recoup value from those fixed costs. It's simple economics.