[justin66]

> Why do people borrow money to buy a car? Why not save money, maybe invest in something safe(ish), and buy your car in cash,

Financing for new cars is often available at rates that are so low, you'd be completely foolish to pay cash.

> preferably used?

Yes, the amazing financing is certainly one of the incentives for buying a new, rather than used, car. That's why the manufacturers offer it.