Others just over-sell and are based on popularity which can wane. Remember "Curves?" They were suddenly in every town - and then nearly half of them disappeared. It was a craze, that apparently took hold and then wore off.
So, there is a real risk, as a franchisee, that you could build up a business, make it successful, and the parent company then drop the ball, or intentionally try to run you out of business, so they can buy you out for cheap.
For this reason, most legitimate franchisors limit the number of franchises for a given area, lest there be saturation problems.
But for the most part, the franchise deals that YOU hear about - on the radio or in the newspaper or on the Internet, are just raw rip-offs. They take your money and leave you with nothing. Serious franchises are for serious people with serious money - not Johnny Punch-clock and his bucket of dreams.
There is no free lunch. There is no quick and easy path to success. If there was, everyone would be taking it!