I factored those. Put them down as opportunity costs, or intangibles. That's a personal decision.
An extra vacation day can have a dollar value - but only the individual could state whether or not they'd take that day anyway and what they would do with it. If your trip goes from weekend to weekend - would you really use any extra time off work? Leave on a Friday after work and get there Saturday afternoon vs. fly out Saturday morning.
If flying, would you just buy an extra resort day with that time and actually spend even more money? What if your trip includes a paid holiday and you have the extra time whether you want it or not? Also some work schedules allow people to just switch days with a co-worker w/o sacrificing any vacation hours to get the extra time.
Based on our trips - it typically doesn't cost any extra vacation time and if travel time were shorter, we'd likely just add time there or down time at home waiting to leave.
For others, the journey down is part of the vacation and they actually enjoy the long drive and the scenery along the way. For others (rational or not), the anxiety associated with flying isn't worth the time saved.
Why would you assume flyers leave on a Sat morning? We leave Fri evening also except we're in FL in time to watch the 11pm news Friday, all comfortably resting. We can go to the parks well rested by park opening Sat am.
You say adding another resort day would just cost more but then we'd have a longer Disney vacation! Is that not an option however you choose to travel?

You're paying more because you're getting more! And those additional park days added to your ticket really don't cost much more. Compare the cost of a 7 day ticket to a 10 day. Those extra 3 days only add $12/day to each ticket.
Assuming the same weekend to weekend timeframe, as a flyer I'd still have the
option to spend the same # nights at a Disney hotel. If I did, just for the sake of keeping # of hotel nights equal to a driver, my Fri night would not be spent in a car, I'd be home in my own comfy bed. Assuming we both leave Saturday evening to go home, I'd also have an extra day at home at the end of my vacation to unpack, do laundry, grocery shop, read mail, etc with my Sat night spent in my own bed, not in a car.
So when you pull up in your driveway with all of these chores still waiting for you, I would be done mine after a good night's sleep.
OR
I could be off the same 9 days, cram it from Fri night leaving home, to Sunday night returning, same as a driver must, but I will have spent 8.5 days in the parks.
In order for a driver to have 8.5 days in a WDW park one would need at least 11 days off. That means 2 more vacation days for dh & I.
They are priceless. That doesn't even factor in how grouchy we'd all be spending so much time in the car or the time spent cleaning out my minivan once we're home.
Also when people gripe about having to arrive at the airport 2 hrs before the flight, half of that time is spent relaxing at the gate, usually on a tablet or laptop, not stressing out over traffic.