Because you are so much smarter than everyone else?
No, just smarter than you.
Often the factory offers these financing deals so that they move cars faster. The Dealer has nothing to do with them, so they do not have to pad the car's pricing to make up for the cost of the loan. I had a major auto manufacturer as a client, and that's what they did. So, while it may not be "free", it may offer a cost advantage on picking that vehicle over another one.
I'm not sure what percentage of the total you mean by "often", but I grant that it may be in part a tool to move models off the lot. Buying a car from a dealership is like gambling. The house almost always wins. That's sales experts/organizations vs. average consumers. That's capitalism. Advertising 0% interest is a fantastic marketing tool. It gets people in the door, where they are immediately at a disadvantage because now they're in "your" house. And they have many tools to extract additional dollars from you outside of the financing.
I just prefer to walk in the door knowing exactly the rate I can get and with a draft check ready to go. The one time I made the mistake of allowing the dealership to offer financing at the rate I knew I could get and with the credit union I knew they could offer financing with, they came back with double the rate I could get, with some other financing institution, and had managed three hard credit pulls on my credit.
No thanks.