Well with 3 kids in school and 2 employed parents (thankfully), we drive 500-600 miles in 1 day to get where we are going. When you only have 1 week off of work and you want to go 600 miles away, you don't exactly want to waste it driving and only have 3 days at your destination when you could have 5. But that's entirely a difference of opinion and everyone has different needs.
I just copied and pasted my old post on a similar topic. I'd love to enjoy the ride and be leisure about it all and limit my driving to 300 miles/day .........but I DON'T HAVE THE TIME TO GO SLOW OR I'LL NEVER GET ANYWHERE!! Whew, that feels better! :-)