If you want to spend your time washing and ironing fabric go right ahead. I'd rather sew.
Let me tell you some of the reasons I don't wash. It's not dirty. It comes shrink wrapped from the manufacturer. Fabric comes with a finish that helps avoid sun fading. Shrinkage is your friend and the batting has a lot more to do with shrinkage than the fabric. Even if it shrinks the bed won't mind. Since it's not clothing that has to hang properly the grain doesn't matter either. Even if it bleeds and something else picks it up it's still a quilt and will still keep somebody warm. I'm not allergic to fabric finishes.
The biggest reason is I don't want to spend my time washing and ironing fabric.