because they are inaccurate.
Have you ever taken your pulse? You have two VERY large arteries running up your neck from your torso to your head. One artery runs up either side. It is the most common place to take your pulse. If even one of these arteries were to be severed, blood would go squirting everywhere. You would be lucky to live longer than a minute if the artery was completely severed in two. If both arteries were to be severed, you would be dead period. You wouldn't have enough time to whisper a final prayer. This is what would happen upon decapitation. A blade would cut through the flesh of the neck, the internal organs (trachea, etc.), the spinal chord, and both arteries......the head would be completely seperated from the body. If this were to actually occur......the person would have two spurting fountains of blood...one on either side of their neck.
The human body holds aprox. 8 pints of blood........with a little more absorbed in the small blood vessels lining the cranium. If a person's neck were to be completely cut in two, both arteries severed completely.........you would have about 5 pints of blood squirting from those two arteries in a matter of seconds.