It IS indeed a combination of lack of skill and not training it. Federer's net skills won him the Wimbledon final this year, he came in more than any final he had ever played prior IIRC..
Regardless, it is truly a lack of skill. Players do not know when to cover the line or when to cover the cross court shot. They are indeed a sitting duck, the courts aren't slow, they are... On top of that, as I mentioned, we do not have an elite player that uses this method, so how can one insinuate that a player cannot get to the top with such a style...that is unless you base your ideology using the careers of Llodra and Stepanek for example. They aren't elite players, but both have had very good careers on the singles tour. Imagine if they had the natural talent of like a Federer, or physique/mental strength of Djokovic/Nadal. You really think that can't make it to the top of the game?
Players get passed because they don't know the proper way to play at the net. Just compare the great S&V players to now. They knew where to cover. Sometimes you have to play the percentages, sometimes you have to take a risk, sometimes you're a sitting duck and have to guess, it all depends and a good net player uses these different approaches. At the same time, while there are good volleyers that don't come in that often, majority of the players don't have the volleys of great net players..they just know how to cut off a floating reply. Once the ball comes back with heat, not too many can deal with it properly.
If you want a little comparison, look at Novak's success last year mainly against Rafa. Djokovic was oftentimes literally standing and waiting for Rafa to hit the ball back crosscourt and all he needed to do was stick his racket out for an easy volley. Then looking at Federer in his prime years against Rafa, he was so stubborn about that very same thing. He would cover the down the line shot time and time allowing Rafa to pass cross court pretty easily.
Just having good volleys is only a very small aspect towards being a great net player. Look at Ivanisevic, he had very mediocre volleys, regardless of whether you think how much surfaces/technology have changed, he was still a top player. Hitting the right serve, knowing where to move, being confident with the play and take the right percentages are much more important factors in being a great net player that players these days don't have.
It is no surpise that players of the current era dontīt have technically great volleys. SInce itīs a tactic that dpesnīt really work nowadays, they dont practice volleys, so obviously when they do come to the net, it looks a little clumsy. But thatīs all due to the slower courts. Why practice something that doesnīt work even if you execute it well?
The same goes for their net coverage. There are no Edbergs or McEnroes because no one practices that style. But thatīs only part of the story. Slower courts and better technology of the rackets have given a huge advantage to the baseline players. Sure, Edberg, McEnroe, Rafter etc. covered the net very well, and had great volleys, but then again, they never faced players like Djokovic or Nadal. Slow surfaces mean that players have much more time to get to the volleys then they used to, and modern racket technology allows them to hit passing shots with such steep angles and huge spins, that the net-players chances of winning the point have decreased substantially. Itīs a very difficult to hit a first volley that would put the player in a winning position. While it is nice to speculate how Edberg-Nadal match would turn out, for example, in reality it is pretty clear. On a modern hard court, with modern technology Nadal would pass Edberg almost as well as he passes Federer, for example. Edberg would volley better than Federer, but it would not give him enough of an advantage. At least, thatīs how I see it.
As for Ivanisevic, his net skills were really not that special. He was a tall guy who had a huge serve and pretty lousy groundstrokes, so the sensible thing for him was to follow the serve to the net. But it was mostly about finishing the points when his serve didnīt do it, I wouldnīt rate him as a real "volleyer", who covered the net exceptionally well.