Why do certain people think they drive better when stoned?

They're just being stupid and childish.

Grown-ups know that intoxication doesn't only come from beer and spirits. Buzzed driving is drunk driving, yada yada yada. But I saw some guys in suits drinking a pint with their lunch, knowing full well they have to drive back to work. Grown-ups also know that 1 beer FOR THEM doesn't really intoxicate them nor would they register a .08 BAL or above. 

Of course if you get done with a full smoke session, you'll be SO HIGH MAN, like to the moon. Those are the times they will go as far far as their kitchen for munchies and something to keep the cottonmouth at bay but they won't try to drive. You however with your totally true story weren't experienced enough to know that and put lives at risk. Idiot. Then again there's also marijuana out there that is one hit and done so it really doesn't compare to beer or even a shot or 2 of whisky.

DWI Stands both for Drunk While Driving AND Drugged While Driving. 

Alcohol and many drugs affect your perception and other mental processes so users may THINK they are driving better when, really, they are not. Prescription drugs can have the same effect so it would be illegal to drive if affected ... even if we think we are OK.