[Piotrsko]

The flares are added to reduce wet spray, more glamorous than the flaps on a commercial vehicle and keep some of the road dirt off the body. In order to get clearance for those wide large diameter tires you need to change the offset of the rim so the tire doesn't hit suspension when turning corners. That change in offset sticks the edge of the tire further out in the windstream. Ever see a 4wd covered in wheel spray gunk?

As for wide enough: not a consumer demand. You can almost put a full sized twin bed mattress flat on the rear seat of my F250, but it's a huge pain to park in most shopping centers between the lines and still get the doors open.