The tempered safety glass used in vehicle side windows is engineered to withstand large force spread over wide areas, but is easily defeated by even moderate force applied to a pinpoint area. I have seen side windows broken in by rocks, hammers and knife butts, but it always takes MUCH more force than you would expect. I've even watched a guy in a fit of road rage punch through a side window, but I think it was a very lucky shot and he broke several bones in his hand. Conversely, I've seen cops beat on windows with their nightsticks and do nothing more than fracture their wrist, and suspects locked in patrol cars kick at windows with no effect.

The problem with using a knife blade is that you're holding it on the handle, far behind its center of mass and fulcrum point, which makes it impossible to control once the tip meets the smooth resistance of the glass. This translates into the blade twisting in your hand and the force is deflected off the glass, and as NobodySpecial notes, possibly into your leg. You would be better off using the butt-end of the knife, but you would need to use a lot of force.

With a punch you can hold it so only an inch or so extends beyond your grip. Because you are gripping the punch on the inside of its center of mass and fulcrum point, the punch doesn't deflect and the direction of force is perpendicular to the glass. The conical tip then concentrates that force to a pinpoint area, making it easier to break the tempered glass than the wider knife butt.