It's often because people line up their car according to the car next to them. If one person does a lousy job and the next person copies them it begins to spread like a virus.
Yeah this is probably it. Would explain why people with small cars always pull in waaaaaay to the front. Everytime I think I see a space I'm bamboozled by small cars.
A good rule of thumb is to look just below the side mirrors. The average height person seated correctly will see the stop line appear just below the side mirrors when the front bumper is at the line.
Easy way to know with yourself in your own car is to park right at the line, then sit in the drivers seat and compare where the line is in relation to the mirrors when seated normally.