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.
