There is a whole list of reasons.
Number one on the list is: a car loses massive amounts of money within the first couple of years. Who do think is paying for that depreciation? Hint: it isn't the leasing company, it's you.
It's a well known fact that as soon as you drive a new car off the lot it loses thousands of dollars in value. Would you pay three thousand dollars to drive a car out of a parking lot? Of course not, so don't lease it.
When you lease a car you aren't renting the car, you are essentially buying a new car and selling it back to the company at the price they are willing to pay.