Understanding NFL Defensive Coverages: A Quarterback's Challenge

Question
why do people say that NFL presnap reads are hard?

Answer
Dear Jordan Kelly,
The NFL teams have a lot of ways to fool a quarterback. When a quarterback walks up to the line of scrimmage, he might see a cover 3 defense. When the ball, is snapped, it might turn into a cover 1, cover 2, a corner blitz, a fake blitz, one of many zone coverages, man, double team... Well you get the picture.

Coach Louis.