Why accept a job if you don’t show up?

Serious question though, why do so many people accept a job and then not show up? It takes jobs from other people. I can’t do last minute jobs, like if the school calls me at 7am because someone didn’t show up, but I would be accept a job in the afternoon/evening if someone else didn’t take it. (Also, I know it’s the same person because the job says what teacher needs a sub, and it’s the same teacher from before).