I mean, if my experiences are anything, it's because people don't care enough. At fat food joints you've got a bunch of high school and college kids who want to get off and party or get off and finish their mounting homework. It's not willful, just part of being 17/18/19 at an "unimportant" job. We had some adult-adults who worked there as well, and it was a night and day difference.
Again, it's not willful or malicious, just laziness.