Because we have no disciple(result of coaching) or coaching..
Fly balls are one of the easiest things to do. How many times have you see out hitters choke up, widen their stances, and don't stride when we have a man on third with less than 2 outs?
The stated approach is what hitters should use to hit the ball where they want to, ground, air, at 1B, at 3B, etc. We never do change our approach in these situations, hence the small village we leave in scoring position every game. It all comes back down to coaching and coaching of the fundamentals.