Fuel leaks (those systems are designed for a pretty constant 1 G), loose objects, different cars having (sometimes radically) different weight and weight distribution, the impracticality of a fastening system (any secure one would either dent the car or require holes drilled in it, never mind the shape of the vehicle), the risk of setting off the airbags, car seatbelts not being designed for this purpose, horrendous throughput... There are myriads of reasons why this wouldn't work. Not to mention that it would be a much better use of the money to just build a car park and a rollercoaster separately.