they're not training it less. They're using it less. And they're not using it less Because their tactics is by default baseline tennis...their tactics is by default baseline tennis Because they can't be successful going at the net too often.
There are days--weeks even--dedicated to volleys at various clinics and camps here, you just won't see it incorporated in the game as much because it's no longer the most proven tactic in tennis.
People always talk about racquet technology and surface changes, but they forget about the guys holding the racquet and the modernization of the game. For all anyone cay say about racquet technology speeding up, remember how athletically sharp these guys have to be to cope with that. Moreover, and more importantly, the mechanics of a stroke aren't what they are even some decades ago.
The predominant ball from the baseline is no longer a flat one, the type easiest for an astute volleyer to redirect. Topspin has really become a staple in the game, meaning not only the speed, but the weight of today's average shots has increased. Volleys are made more difficult to accomplish because of how well guys can arch shots at such speeds and trajectories that wren't as much of an issue before.
Just remember even Sampras' comments about Hewitt being a victim of his time. The greatest serve and volleyer of his time said Lleyton had the game to largely break that style of tennis, but was unfortunate to come on to the scene when the dynamics of the game had changed.
It's not a matter of lack of practice making for poor results, it's a matter of (mostly) poor results making for a lack of implementation.