Bunting down the third-base line (for a lefty) isn't necessarily as easy as it sounds unless you're, y'know, GOOD at bunting. You have to bunt it flat (not up) and have about a 45 degree window to bunt between the pitcher and the 3rd baseline.
And that takes practice. But I'm not disagreeing... I'm just pointing out that it would take a paradigm shift, followed by practice (cue Allen Iverson tape), and ending with proper execution.
The reason many don't do it is the same reason that ****** free throw shooters don't shoot underhand (a proven easier way).... they'll look stupid if they fail. One bunt popped in the air or bunted right back to the pitcher will make them look foolish, and that's a risk they won't take. Even a foul bunt or two might loosen up the shift, but to most, the risk is too great.