Proper is a Good Thing, No?

One thing that bothers me about my parents is how they always rant on dinners at home being a casual thing.  It might be cultural, but I don't think it's always the case.  I think "dressing for dinner," even if it takes place at home is a good thing, once in a while.  I have always worn dresses or skirts - or at times, nice pants - to Thanksgiving or Christmas dinner.  My parents think it's ok to wear jeans, and maybe even sweats.  Isn't this  WTF situation?  I understand the need to be casual, but really, sweats?  When you have guests?  When it isn't a backyard barbecue?  Of course, I've seen my parents wedding reception home movie.    It looked like a big fat regular dinner out to me.  Le Sigh.