What contributes to the lethargic feeling when riding in a car for long periods of time? could inertia play into it? looking for the scientific explanation of how movement affects the body.

Exactly the same reason babies' cradles are built on curved runners, for rocking, and why we put the baby in a swing or rock the baby to sleep in our arms.  We are hard-wired to become soothed and relaxed by gentle repetitive motion.

But that still isn't an explanation of "Why".  My own understanding is that this is a carry-over from the earliest stages of our brain formation, when we were suspended in a pool of warm fluid in utero and our mother walked around during the normal activity of her normal day.

In my own personal case, I could never be an aircraft pilot.  As soon as my seat-belt is buckled and the airplane starts bobbing up and down, even while still on the ground, I am out cold.  I seldom remember take-off.  I have been on commercial business flights, that lasted 18 hours and I slept through 16 of them.  Turbulence in flight is even better.  If I happen to be awake, flying into a storm will knock me out in seconds.