So a couple of things, it usually isn’t the outfield where they shift (well they do, but it isn’t as dramatic) and not where the ban is. The ban is for the infield. For a right handed hitter, the 2nd baseman will shift over and play on the 3rd base side of second, and the SS and 3rd baseman move over a little as well, basically saying “we know if you hit a ground ball, you will hit it to the left side of the infield and there’s no way it’s going to get through us if you do now.” For a left handed batter it’s in reverse and they all shift to the right side of the infield for the same reason.

Asking “why don’t they just hit it the other way” is sort of like saying “why can’t a pitcher who hurt his elbow on his throwing arm just pitch with his other hand then?”

First it’s not every batter they would do this for. It is only about 40% of batters, those that overwhelmingly pull the ball. These batters have hit like this their entire lives. Their entire approach at the plate with timing, movement, rhythm, muscle memory, etc is predicated on hitting that way. And remember that these are basically the best people in the world at hitting a ball with a stick. Could they learn a different swing? Maybe, but they made the major league because of their current swing.

Also, keep in mind that when the defense employs the shift, pitchers pitch a certain way as well. Batters tend to pull pitches on the inner half of the plate and push pitches on the outer half. And a batter will tend to pull off speed pitches more and push very fast fastballs (like pitches that can hit the upper 90s and triple digits). Finally higher pitches are easier to hit in the air, and lower pitches, especially those with late breaking movement tend to be hit on the ground. So a pitcher will throw primarily on the lower inner third, and will take a little off their fastball and throw off speed to get the batter to pull the ball, and will try to throw with downward or inward movement to force grounders and pop ups.

Changing their swing to try to hit the opposite way could have a drastic impact on their hitting ability in general, and ruin their ability to return to their old swing when the defense isn’t in the shift. Plus for most of those hitters, the goal isn’t to hit a ground ball anyway. The goal is to hit a home run in every at bat. The shift doesn’t matter when the batter crushes the ball. Changing their swing means they aren’t crushing the ball anymore. So most of the adjustment they are doing is swing lower since they will be expecting pitchers on the lower inner quadrant and are trying to avoid hitting a grounder.