Because that tennis player doesn't ALWAYS hit the desired return serve. And that 3rd baseman doesn't ALWAYS snag a line drive from the hop and nail a perfect throw to 1st base.
Some pool players, like myself, don't need kicking systems. I do an excellent job kicking 1, 2 or 3 rails simply by looking at the shot. I'd say I'm 90% efficient and consistent at getting at hit on kick shots. That means I don't ALWAYS get a hit. If I used a system every time, maybe my success rate would go up to 95% or higher.
Good pool players do whatever we feel makes us more consistent. Those who don't probably lose more often than they win. If you feel ghostball works best for you, do it. If you just see and do, then do that. That's what I do, with exception to a few shots where fractional aiming (Poolology) comes in handy. If a player up the road feels like contact points works best, then that's what he's going to do. And if another player feels CTE works best, then that's what he should do.
I look out for my own interest, my own game, and if anyone decides they don't like the way I do it, I really don't care, because my game is about my preferences, not someone else's.