Because it acknowledges how blessed you are to have that system in place to be able to go get clean food and eat it and be sheltered. Yeah you worked, but you're healthy too. It's gratitutde that makes life better. Instead of complaining about what you lack, give thanks for what you have. It's a mindset of peacefulness.

It makes more sense if you give thank to "the food" you are about eat as it will keep you alive. This means you should thank the chickens or pigs or cows rather than give thanks to someone who gave you the hunger in the first place.