Would you drive 3 days to go to Las Vegas and what's the reason of not flying?

I would probably drive. My arms get tired when I fly.

It'd likely cost you as much or more in gasoline and lodging.

I personally would not. But I know people who have made the drive from the east coast to Phoenix, and really enjoyed the trip, as they saw many different kinds of scenery, and got to know a lot of different states. There are people who are inexplicably afraid of flying (you stand a much better chance of dying in a car). 

I personally probably would not, unless a stop in Vegas was part of a road trip.  But I do know people who make drives like that for their vacations, often because they cannot afford to fly.  Some people might be scared of flying, some people might not want the hassle of flying, some people might want to have their own car available during their trip, etc.  There can be a lot of reasons for making a long drive for a vacation. 

I would drive 3 days to Las Vegas if it were to sightsee along the way.  There are some wonderful places in the Southwest.  If I weren't sightseeing, then I would have no desire to drive that far instead of flying.  For some, however, they may have issues with flying or with the expense especially if they have a full car of people to share the road trip expenses. 

My inner ear makes flying quite uncomfortable for me.

But, I have no interest in Vegas. I have driven across the continent for other kinds of events, literally coast to coast.