Well - first off - they DON’T. Not all the time. If I look at a street of people and count how many are doing the classic swinging of BOTH ARMS I see about 1 in 10 people doing it.
This are my guesses as to why we do it:
- as a kid, as we learn to walk, we do it to balance. We keep the habit as we grow older
- when we were 4 legged apes we walked on our arms as well. it’ a genetic leftover.
- we do it cos we see people swing their arms as they walk
- the arm acts as a pendulum. It’s about the same length as the leg.