I played center and most of the time in shotgun, I looked up several times. Defenses don't tend to show their blitzes until the ball is close to being snapped. So I had to look up and back down to the quarterback more than once. So I didn't snap it every time I looked up. Also, the quarterback normally had a signal for when the play was finally ready and I'd look up, wait a second or two depending on the count and then snap it. If you don't look back at the QB it's really hard to just snap it on touch hoping it gets to the quarterback. Also, a lot of these snaps aren't on a hard count, but a second or two after he lifts his leg once and then twice sometimes.
Also, yes, if the snap count never gets changed, which happens a lot unfortunately when the offense is trying to get into a rhythm, the defensive line is always taught to look at the ball. If you look at the QB, you'll probably be a step behind when the ball is snapped. At the same time, if you don't change the count, the defensive line will start to fire off like its a race and the count may as well be "ready set go" every time.
So, most of the time the center looks up several times, not once, and calls out the blocking assignments while reading the blitzes. I don't know if I explained it well but I hope this helps.