And how does falling asleep imply relaxation? If anything, falling asleep implies mental fatigue. So we can assume driving adds more mental strain. There are entire studies done on driver visual search patterns when driving [0]. What kind of visual search pattern do you use when walking? Are you constant scanning the horizon looking for potential hazards? In fact, walking is generally considered to be an automated activity [1].
[0]: https://pubmed.ncbi.nlm.nih.gov/16127339/ [1]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4879139/
And yes, walking is not stressful for most people, that's the point of my first reply in this thread. There are people that find being outside as stressful as you find driving if not more, it's a real condition called agoraphobia. It's silly to pretend everyone is affected by such a condition though and trying to rationalize this is not strengthening such an argument IMHO.