Why?

Why do parents send their kids to school with the statement "call me and I will come get you if you don't feel well"? At that point, just call out from your job and keep your child at home. I realize that it is probably said in frustration as the last sentence in an argument to get them to school, but come on, you are telling them to cut the day short. This has become an almost daily issue when at least one kid comes in with this info, stated in an expectant tone as if the parent is like a firefighter waiting for the call to spring into action.