I went to a drive in theater a few months ago with some friends. It was a pretty terrible experience. We tried sitting outside since it was be better than sitting in the cramped back seat of a sedan. That made it impossible to hear most of the movie since other cars would turn on their engines, or a low flying airplane would pass overhead, or a train would go by. Other problems: light pollution from cars driving by with their headlights on would fade the screen, other screens playing different movies was very distracting, and going to the bathroom would take 15-20 minutes since it was so far away.
A car seat is never going to be more comfortable than a movie theater recliner, or your own couch. And listening to a film over the radio isn't going to produce as good a sound as your home theater system.
It sounds like you were simply looking for a different sort of product. The point of a drive-in theater is not to produce a high-fidelity experience. You may as well go skiing and complain about the cold.
A car seat is never going to be more comfortable than a movie theater recliner, or your own couch. And listening to a film over the radio isn't going to produce as good a sound as your home theater system.