Yep. In every company I've been in here in the UK, I have pretty much never heard of an ex employee returning to explain their work, fix a bug, do training, etc. And well, the more people leave a company, the less likely they are to come back to help at all, cause of a 'toxic' atmosphere or sub par working conditions.
I think pride plays its part here. People want to prove that they could indeed cope without the person. I think there is also the point where people think, "we are going to have to learn this stuff one day, it may as well be now".
In some key-person areas long notice periods (3months, for instance) are the norm. However this is never going to be the most productive 3 months of anyone's life! Serving 3 months notice becomes very dull by the end...
