It's just one of the fun quirks of baseball. Back in the day it was a function of baseball being played on fields meant for other sports (eg. the Polo Grounds) or crammed into the space available (eg. Fenway Park). Then in the mid- to late-20th century, they did build a bunch of cookie cutter ballparks with curved, symmetrical outfields, domes, astroturf, etc. Those were awful, soulless, monstrosities.

Then when Oriole Park at Camden Yards was built in '92 it brought back a "renaissance" of these ballparks that were part of the city. They used existing architecture, borrowed architectural motifs, emphasized skylines, etc. So really it's partially about nostalgia and tradition and partially about emphasizing that each park, city, and club is 'unique".