Wel saab was a airplane manufacturer until it went to make cars for a while.
I'm thinking Buckingham did something similar.
Since they make modern aircraft like the Luxor and we have never seen a car by them it's most likely that...
A. They made cars a long time ago. Look at the font on the window, looks like that old style you see associated with classic aviation. That can mean that they made cars in the past (possibly alongside their aircraft) and have stopped to focus on aircraft and have since become insanely rare and collectible, since none are on the road.
B. They make new cars, but are sold in limited numbers. The font could be a reference to their origin from making aircraft years ago and the reason we never see any of their cars is because of their high price/extremely limited numbers.
Then again, there were supposed to only be 40 Stinger GTs and 10 Z-Types ever made, but we can buy those.
I've heard rumors of an aviation themed update and since there are usually new cars with the updates it would fit in with the aviation theme to put in some cars from a company with a background in aviation. Might see a Buckingham made car one day.