Why is everything closed?

I don't know if you are aware of this little tradition in France and what seems like the rest of Europe for that matter, but on Sundays everything shuts down....completely!  I did not know this coming into my study abroad experience, at least I don't recall if I did or not, but it can tend to be slightly bothersome.  Lets just say for the sake of argument that it is Sunday afternoon and you just left the gym and you are walking home...you are thinking to yourself, "Hey, I would love 'insert favorite food here'.  I have a great idea I should stop by the grocery store on the way home and buy it"...WRONG!  You would not be able to buy whatever food or drink or anything purchased in a store for that matter because everything is closed on Sunday...even the usually handy grocery stores.  I understand that the French like their time off and vacations, but for everything to close on Sunday is a bit severe.  Can't they at least leave the grocery stores open, if not for the entire day then for a select number of hours?  What if a mother(or father) needs to get food for their baby?  Or some of the nasty ass milk the have here for breakfast?  There are countless necessities that someone might need on a Sunday that they will not be able to get over here.  

This is just one of many observations that I have made while in France and one that ranks high up on the list for most intrusive on my everyday routine...but only on Sundays of course.  I guess what we can all learn from this is that you need to think ahead and be prepared for the worst on Sundays...because if the worst happens, you better not be in France.