Lambert's resignation

Now, I accept that Lambert's imminent resignation is not related to the Liverpool board, but I'm pretty sure the population of Norwich fans on here is low. Anyway, this is not necessarily about Norwich or Lambert, but more about football managers in general.

Why, when any offer comes in from any club, do managers always seem to leave? Obviously a lot of these are understandable - Rodgers to Liverpool, AVB to Chelsea, Redknapp to Tottenham. These guys left clubs at which they were heroes because they have ambition to go on to bigger and better things at bigger and better clubs.

But with Lambert - why? In what way are Aston Villa superior to Norwich? I don't care that they've won leagues and a European Cup whereas Norwich haven't, because that was all in the past. That would be like Pardew leaving Newcastle for Liverpool - just because they were once better doesn't mean they are now.

Some people say that maybe Lambert just wants a new challenge. Which lacks all logical merit. Lambert has settled in at Norwich, and how. Having lifted them from the bottom of League One to mid-table Premiership, he's practically a deity in Norwich. The fans adore him, the chairman loves him and the players clearly have 100% respect for him. Why would he throw that all away for a so-called 'new challenge' that may well ruin his reputation and blemish his CV? I cannot see the logic in it.

I understand that there is not much to comment about here, and that I may have ranted on. But when even managers can't seem to stay at a club a few years without jumping ship, what does it say about the state of football?