IMO #1 reason for shorter casts is line control. Fishing dries (and dead drift nymphing) you need to have control of the line on the water so your imitation looks as natural as possible. Most (not all) real bugs pretty much drift with the current. Whether you’re targeting a specific riser or casting to likely looking holding spots if your fly is dragging you probably won’t get a lot of attention from trout.
Where a dead drift isn’t important (lake fishing, fishing for steelhead or salmon, etc) the ability to cast a long line is an advantage.