If a DB isn't looking for the ball, he's not a good db. Period. Can't stop what you can't see. I have seen a lot of it too. It is a known that if you don't look back for the ball a pass interference is coming and it's an easy call, and DBs are still playing the man instead of the ball. I see it all day on Saturdays and also on Sundays. The fundamentals are not taught anymore.
Why is this happening? Probably because of your statement about DBs being a dime a dozen. If coaches feel that way and want to just convert any athlete to corner, then that is a lot of the problem. It's hard to find a true cover corner with the ability to backpedal, turn, and run anymore. One that is confident in their closing speed and one that has the confidence to turn their heads, while still running with the man. Corner is the one position you should not just throw anyone at. It will get exposed.
If you have corners playing 10 yards off of WRs, you are pretty much giving up 5 yards automatically the way some of these QBs can throw. You are not a good corner if you are playing 10 yards off. You are scared of getting beat deep and have no confidence in your quickness or ability to recover.
A D line and pass rush is very important, but a weak corner will still get exposed even if you have a strong pass rush. It's called 3 step drop or timing patterns. If you have a corner that can play close coverage or even bump and run, it will throw the timing off. QB hesitates and theres the sacks. It works hand in hand. It's rare a team has two great cover corners. A team usually has one, and then the saftey helps the other one. If both safties have to worry about the pass, then you have big problems. Most teams do not have the luxury of stopping the run with just the front 7. That 8th man in the box is the run stopper, and that is usually the strong safety. If your strong saftey is playing pass first all the time and cannot come downhill to shut down the run, then you will continue to have defensive problems.