The reason that riding in a car for a long time makes you tired is usually because of the constant vibrations of the vehicle as you're driving down the road which then transfers to your body.
Vibrations naturally make our bodies feel sleepy so it's why we usually feel tired when on a long car trip.
You should make sure you get plenty of rest the night before you head out on the road trip and pull over every 1 hour and 30 minutes or at least every 2 hours to avoid driver fatigue.
The passengers falling asleep is okay but not the driver so if you're driving and feel tired you should pull over and rest for 20 to 30 minutes or an hour and take a nap before continuing on your road trip.
The motion of the car can also help us feel sleepy or even the air conditioning or heater can cause drowsiness after being in the car for too long.
Driving is also work so your body gets tired of doing the work required to drive the vehicle.