Because the defense can't gauge where he is going or what he will do with the ball - play action is just not as effective in the shot gun - and less effective in the pistol - as compared to the movement that occurs when the QB rotates after taking the snap while under center.
If the defense can't see, the QB is more dangerous.
Having said that, I recognize that playing under center is more difficult for a QB.
I suspect that we see young QBs playing at a higher level in the NFL because the shot gun makes the position easier to play.
In the old days, the conventional wisdom was that it took QBs five years to master the position in the NFL.
Now, we see rookies playing really early and really well - Joe Burrow, Kyler Murray, Tua, and Lamar Jackson.
I believe these young QBs are effective early is because they are not being asked to master the position in the same way that Terry Bradshaw or Peyton Manning had to master the position.
Just my opinion.