Sightseeing is unbounded. Your losing 40 hours driving at reasonable speeds from SF to NY before considering detours or extended stops.
As such 5 days is just a reasonable baseline. It lets you see the country along the way, have some slack, and show up without being exhausted. Got an extra day, week, or month and you can find plenty of things to do along the way, but you still need to account for the underlying distance. Alternatively, if you’re in a hurry there’s plenty of ways to cut that down.
For me the reason is I was moving cross country and had a dog I didn’t want to put in the cargo of a plane.
If you're spending all day driving, might as well fly?
Though, there are other reasons to do it, like if you're moving stuff along with your car.