It’s cause it’s approached as a sport, in which you usually want to have an even playing field metaphorically and literally.
Football and tennis would arguably also be more “interesting” (a better word is probably “varied”) if there were random playground elements on the field.
Can’t quickly think of ANY sport where it wouldn’t be the case, actually. But then running 100m in a straight line wouldn’t be that anymore.


???
I feel like my point was missed a bit; let me repost a part of your comment with one detail (the sport) changed:
Does that illustrate better what I am getting at?