I suspect that one of the reasons is why we also have the closed questions appear on the main page: checks and balances.
When a subset of the 3K+ users decides to close as offtopic and migrate a question, a different user may disagree and could come here to complain.
And then there's also Theo's argument: if a newb comes here and asks an off-topic question, and sees that it disappears from the front page completely. Which of the following do you think he would do?
- Come ask in Meta about "Why my question disappeared"?
- Look up his User Page to double check where the question is?
- Assume there is a software glitch or that he did something wrong the first time and re-ask the question again so it would show up?
In a generation brought up with interactive adventure games, I don't think 3 will be uncommon.