Once, Twice, Three Times a Christian

My family prays before meals only on the big, feasting holidays:  Thanksgiving, Christmas, and Easter.  Here, in the South, I see people praying before each meal.  (Let's assume that most people eat breakfast, lunch, and dinner, so that would be three times each day.)

I presume they're praying to their god, thanking him/her/it for their food.  What I want to know is why they wait until they're seated at the table before praying.  Those of us sitting with or near them feel awkward if we tuck in while their heads are bowed.  Do they not know they're thankful until they see what they're going to eat?  Can they not pray while they're moving, such as when walking into the restaurant or selecting the food in the buffet line?

Do they also thank their gods for the people who cook and serve the food?  For the people who wash the dishes and cutlery afterward?  For the plants and animals that make up most of their food?  For the scientists who invented chemical additives to deter spoiling?

Do they also thank their gods for the job that they have?  For the car that they drive to work?  For indoor plumbing?  Why limit it just to the food on the plate in front of them?