This has nothing to do with lack of training, and everything to do with how the game is played nowadays. Depth, speed, spin and angles. It is much easier to hit a passing or to hit a damaging shot which not only neutralizes the player at net but puts him in a defensive position, forced to hit volleys off spinny dipping groundstrokes. Its the movement on the ball thats the real killer. Players 20 years ago didn't have to deal with this, the path of the ball was much more predictable and easier to volley off of. That's it, simple.