Can anyone hazard a guess as to why the entire rear wheel wells are not enclosed? Specifically, the tires can splash up road debris (e.g., rocks, salt) directly on to the mufflers causing them to need replacement much sooner than if they were protected from the elements.

 

My guess is production costs, aerodynamics, freedom from ice accumulation, etc. The mufflers are stainless steel and will be long lived. My '99 had the same design and after several years it all looked fine back there.