Because no matter which way I'd have bought the car, I'd have been either saving up in the years previous, or spending as I go after purchase.
In theory for a car purchased within 12-24 months, this figure should be pretty similar either way unless you happened across a windfall of some type outside of your annual wage.
That's the way I look at it - no matter how long you have the car you will buy another at some point. Ideally to keep comparing apples with apples (against those that are financed/loaned) you'd spread the purchase cost over either the time you'd owned the car or the time you anticipated owning it...
....Unless the next car is either free or paid from outside of regular outgoings.
But tbh, it was too much of a pain in the rear for me to try and figure out, so I didn't bother, allowed for depreciation on both cars and just took the interest from the loan on the second car.