Why are subs such a damn crapshoot?

I have been teaching long enough to know when I have had a really good sub in my classroom and when it's someone who is just collecting a paycheck until something better comes along. Here in NashVegas that usually means when/if their country music career takes off, or when one of the songs they wrote gets picked up. I swear you can't swing a dead cat in this town without hitting someone who is trying to be a songwriter or singer or guitar player.
I had to be out three days this week with a sick child, flu in a 5-year-old really sucks. I was able to get a great sub for Monday, she followed the lesson plans and the kids work was excellent.
The sub I got on Tuesday however, wasn't so good. She was the classic, "I'm going to put everything on the board and I'm done. " Thanks, that always leads to excellent work from the students, and them getting the help they need. Come on, I don't expect much from a sub, assign the work at intervals during the day, I tell what time everything is, and let the kids work on it then and help them. It's not too tough.
Wednesday we had a snow day so I didn't have to worry about who was keeping my chair warm and making sure it didn't fly off into space.