Also, we are talking 6 hours, not "a few". And depending on your height, seat recline, and problems with lower back pain, you can bet for some people, 6 hours unable to stand up can be quite painful. Not, "bamboo shoots under your fingernails painful" but not something anyone would want to go through.
For me, the claustrophobia of not being able to stand up would be quite terrible.
Getting out of the car and getting the blood pumping makes me much more alert for the next hour or two. At first I didn't want to think about that meant about how alert I wasn't for the previous hour. Now it's kind of hard to ignore.
You see so many people in interesting jobs that have an origin story of something that happened to them, a friend, a sibling or neighbor when they were a kid. I just write software, so my stories are perhaps a bit less compelling than the oncologist down the block.
Stop the damned car and let your family out for fuck's sake.
Just because you can drive >4 hours nonstop doesn't mean you should drive 4 hours nonstop (I did on a solo road trip earlier this year. I forgot a piece of equipment and didn't realize until I was 5 miles out of town, so I had to 'make up time'. It was still a mistake.) Complaining that you can't drive 6 hours nonstop is some form of insanity that I am not qualified to diagnose.