Because after a car has a few kilometres on it, it is no longer new.
The last car I bought was 1 year old. I had 30 days or 2000 km to decide if I liked it or not. I also took the car on a 200 km test drive before buying it. That was part of the research I did on the dealership before plunking 10's of thousands of dollars down on something.
To take your question on step further, why can't I decide not to buy a house after only living in it for a couple of days?
