Park position flips a lock into the gear ring to ensure the gearbox doesn't move. This achieves the same effect as leaving a manual gearbox in gear. In a manual gearbox, the engine holds the rest of the gearbox in place, but in a traditional auto gearbox, the whole mechanism can still move when the engine is stopped, that's why the lock piece is needed.

People relying on the gearbox lock to keep the car in place is the weird part - it would be like engaging first or reverse gear on a manual car, and trusting that to keep the car in place. Even if the manufacturer showed that the engine vacuum is enough, people still rely on the right tool for the job - a brake.