First, I'm not sure that most groundstrokes, even fhs, are hit in the air. Do you have statistics for that claim?
The reason that guys are going into the air is because they are jumping (hopping might be a more accurate term) into the shot. This is either to put more power behind the stroke, to lower the strike zone, or to get into position quickly, or some combination of the previous.
The most common technique is to load your hitting side leg and jump or hop strongly off that side to initiate the stroke. Nishikori is one of the few guys who will do a "lay-up" type move in which he drives the knee of the hitting side leg into the air as he jumps and then kicks back as he hits the ball.