system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why don't all MLB baseball fields have the same dimensions?
gpt
There are many dimensions that are the same, such as distance between bases, distance from the pitcher's mound to home plate, height and slope of the pitcher's mound, the minimum distance to the outfield fence, etc. These standardized dimensions are the most important to the actual play of the game.

The areas where fields are different can be broken into two categories: foul territory and the outfield fence. Both came about in the early days of baseball due to the fact that stadiums had to be located in less-than-perfect locations, and flexibility in fence locations could help equalize fields on different shaped/sized lots (the "Green Monster" outfield wall at Fenway Padk, for example is very tall to make up for the fact that it is rather close-in to home plate, which is due to the fact that the stadium was constrained by a street on that side when it was built). Foul territory was influenced the same way. Odd-shaped lots caused there to be areas where the stands were very close to the a tion, but the strict 90 degree geometry of the baseball diamond sometimes created gaps between where the stands were and where a grandstand that foloowed the property's contours might be. 

As baseball became an ever-larger business, these designs became more about local history or creating advantages for the home team (an unusual corner in an outfield fence can cause a ball to careen in a direction a home-team player could anticipate, but that might surprise a visiting outfielder) but there is no outcry for dimensions to become more standardized--in fact, these idiosyncrasies are seen as adding to the charm of the individual parks--so stadiums continue to add their own character through unique outfield and foul-territory dimensions.

These playing differences can be attributed to two simple quirks of baseball. First, the area out of bounds (foul territory, or the area not between the first and third base lines) is still a usable part of a baseball field. And secondly, since offense and defense are played in turns, but by the same set of players llaying both sides, a team can be built according to strengths that take fhe park shape into consideration. 

For reason 1: a ball hit into the air and caught in this foul territory counts as an out, so it's construction makes a difference in how a team is positioned and how batters approach their at-bats. On fields with a lage amount of foul territory, more pop-ups will be caught for outs. A pitcher might be more willing to trade a small chance at a long hit in order to try to cause the batter to hit a pop-up into foul territory for an easy out, while the batter will not swing at as many close pitches to avoid fouling a ball into the air in-bounds that may be caught for an out.

On fields with small foul territories, a pitcher won't be able to rely on a as many pop-outs, and may pitch a batter in a way to cause more ground balls to be hit. Batters, on the other hand, will be able to swing more freely because it will be easier to deflect a close pitch into the stands rather than into the air above the in-bounds foul territory of the playing field.

For reason 2, a team with a large outfield can afford to spend less on pitcher's because lesser-quality pitching will be masked somewhat by a more difficult distance to hit a home run. This team would spend more of its budget on fast outfielders who could not only cover the larger area and reduce doubles and triples afforded by the larger dimensions, but who could run faster when batting to stretch singles into doubles or doubles into triples in the larger outfirld, against players no accustomed to covering such an expansive area.