There are a lot of very, very, VERY good hitters out there, and the MLB scouting community gets to see all of these excellent hitters. And hitting is a high priority...not walking. Now if there are two equally talented hitters, and player x has a better concept of the striking zone than player y, then x gets the square.
Knowledge of the striking zone is desired, but not nearly as great as the ability to hit, hit with power, control the barrel through the zone, etc.
"You can't walk your way off the island."