Saying Grace?

I've always believed that saying grace to thank god for your meal was disingenuous (not to mention a waste of time) because when you say grace, you're not thanking those who're truly responsible for getting your food to your table, such as farmers, truck drivers, processors, grocery store staff, etc.

On that line of thought:
Because saying grace is disingenuous, and possibly dishonest because people know that farmers grow the food, then is saying grace a sin? Probably not, since sin is a stupid concept anyway, but it still doesn't change the fact that saying grace is disingenuous.