Why are so many good hitters poor defenders (at Non-Shortstop Positions)?

Hitting a baseball is the hardest skill for position players, yet many players good at hitting but remain below-average defenders, even at positions that are generally viewed as the least demanding—such as first base or left field. These positions typically require less range and speed. If these players have mastered the far more difficult task of hitting major-league pitching, why can’t they at least develop into average defenders at relatively easier positions? What prevents strong hitters from becoming competent fielders, even with years of professional coaching and practice?