If the Cornerback is playing the flats and trying to get under the route, the receiver will convert his route to a Corner so he can get to the open space.
One of the hardest things to do as a play caller is to find a way to get the ball in the hands of your best receiver. Many defenses will provide a variety of looks and shifts to take the best playmaker away from the offense and force them to win with their less talented players. This leads to a cat and mouse game between the offensive and defensive coordinators to see if the offense can get the ball in the hands of their best athlete.
Modern coverages take one of two approaches to confuse the Quarterback and Offensive Coordinator. Some defense’s will align the same on almost every snap but morph into different coverages after the snap. This stops the Quarterback from making simple pre snap reads. The other way defenses like to keep the offense off balance is by presenting a variety of different looks pre-snap.