The way you stated it, it sounded like you wanted to take Nellis from the airport, which is impossible. 

 

I know what you're referring to. If you're traveling east on Tropicana, you have the opportunity to cut north on Nellis. That will ostensibly save you about half a mile.

 

But here's why the evil intransigent cab driver argued with you. First, that route leads you squarely into the busiest intersection in that part of town: Nellis/Flamingo/Boulder. The traffic is always heavy and the signals take forever. Then, you're north of the entrance to Sam's, so you have to drive south on Boulder, make a U-turn, and double back. It would have been far easier to continue on Trop and turn left on Boulder. Straight shot to the casino entrance.

 

I know of whence I speak. I used to work there. I've found that Google Maps doesn't take stuff like this into account. The number of times it's led me all over hell's half acre are too many to count. I'll bet cab drivers absolutely hate it when the turistas in the back seat whip out their phones and tell them what the "best" route is.