Because I prefer the flatter look and wish to avoid surprises later.
I also want to know - approximately - what the finished size will still be after the completed item has been washed.
I feel that it is prolonged agitation that makes fabric look "used" more than dunking it - so that is why I soak it first - and then give it a very quick gentle wash. I also do not crowd the dryer when drying the fabric.
Also - if I had a baby quilt and my little one had just anointed it with pee, poop, or puke - or kool-aid - or blood - my first thought would be to get it washed - and I would not be thinking about color catchers, Synthrapol, or any other special treatment
I do shudder when I think of bleach, though.