Because it will save me money if they don't come?
DS not bothered either way about these children (was more a polite 'you invited me, so I'll invite you' sort of thing), and I'm nothing more than a playground acquaintance to the parents.
I'm quite happy to do a 'Oh, we didn't think you could make it! It's fine, I'm sure we can sort something out...' act if they do just turn up, so I'm not bothered about them arriving unexpectedly. I just don't feel like nudging them.