I've noticed this too. There's two factors at work I believe.
First, I don't know if the equipment rules are the same in the NFL as they are in high school and college. However, it's worth noting that many NFL players are very lax about making sure both snaps on their chinstraps are fastened. If this is a rule then it needs to be enforced.
Second, I notice a lot of NFL players who wear their helmets loose for comfort. Both loose on their heads and loose in the chinstrap. I notice this mostly on quarterbacks, running backs, and receivers.
If the NFL is truly interested in protecting players then they also need to be more strict on the enforcement of the mouthpiece rule. The NCAA is bad about this too when it comes to quarterbacks. From what I've read, a mouthpiece is important to helping prevent concussions. Instead of having all of these rules that hamstring the defense on how they can hit the quarterback, how about making the quarterbacks wear proper equipment including a mouthpiece.