I think a lot of it has to do with being target conscious. When we throw an object at a target, e.g. a football, we naturally focus on the target visually while we throw it. But we're not doing that in tennis; in tennis, we're hitting an object with a racket. You're hitting a stationary, fixed point in space with your racket, just like you would if you were swatting a fly, or driving a nail with a hammer. How do you drive a nail? You focus visually, very intensely, on the head of the nail. If you don't, you will miss it. It's the exact same principle as hitting a golf ball. You're not swinging the club at the green 400 yards away; you're swinging at a little white ball down on the ground in front of you, and if you look it away from it for a split-second, you'll miss the shot. Golfers go to far away workshops, spend thousands on lessons and custom clubs, etc., but, because they don't focus visually on the impact point as they swing through it, they spend their lives scattering balls ten ways from Sunday.
When you think about it, it's logical. How do you hit a stationary object without looking at it?
Of course, the best example is Federer's forehand. Watch his forehand on YouTube sometimes. The ball has already crossed the net, and he's still focusing intently on the point of impact. Same with his backhand and serve, for that matter.
It's really astounding how many beginners struggle and struggle for years with lessons and techniques and practice, and, like the golf duffer who spends thousands on lessons and still can't break 90, still can't hit the side of a barn because they disregard this one, single, simple critical element of a stroke.
An interesting anecdote on this subject.... on a clear, sunny day a couple of months ago, I was practicing my serve about 1:30 p.m. On one end of the court, I saw a clear blue sky, but on the other end, the sun was in EXACTLY the same spot as the ball when I tossed it. It was just BLINDING; I couldn't even see the ball. Impossible, right? No, it was just the opposite. When I served from the "blue sky" end, I did OK, but hit some duds, nothing special. But when I served from the "blinding sun" end, wow!!! ... I hit one fantastic, blistering, cannonball serve after another. WTF? What happened was that the blinding sun forced me to focus as intently on the ball as possible - I had to, in order to see it - and as a result my serves were absolutely dead on. An interesting lesson.