Different position = different ready position
Here is a great picture I found online that shows three different position players in three different ready positions on the pitch.
Third base. Because the distance to the batter is shorter as a third baseman, a grounder has a better chance of getting to you more quickly. Add the fact that batters tend to hit with more authority to the pull-side and you now know why people call third-base the “hot corner.” As a result, third basemen generally need to get lower on the pitch to avoid hard hit balls from getting to them/under them before they can get their gloves down. Less range is required for third basemen since the line is close to their right. It’s more of a 1st step reaction position and therefore third basemen need to be lower and ready to explode. The same goes for 1st basemen on the other side of the field.
Shortstop. A lot more range is necessary for a shortstop. The ball also takes a little more time getting to them. As a result, shortstops can stand more upright on the pitch in order to be able to cover more ground around them. If they were to start lower like a third basemen, they would have a harder time getting started in order to cover more ground. The same thing applies to 2nd basemen on the other side of the field.
Outfield. It doesn’t make sense for outfielders to keep their hands out front because they are not infielders. They also don’t want to get lazy and put their hands on their knees which will make them slow. So where do they put their hands? Like the picture shows, outfielder hands go on the front of their hips on the pitch. It’s a place to put them without taking away from their first step quickness and their need to cover large distances in the outfield.