The reason is simple. You don't create wealth by spending money. Wealth is created by producing something of value. Yes, you'll help keep the car salesman employed. But if you borrow money that you'll struggle to repay, you've actually become a burden to our society.
More debt makes our society weaker.
If you have too much debt you can't afford to help others. You'll be in a worse position if something interrupts your income. Then you could end up asking the government to help pay your bills and become a burden. She can ask herself a few questions before making any purchase. One final thought.