Bad Manners and Brimstone

The days when anyone expected to join a firm on qualifying and work there till they retired are long gone. These days it can be an actual strike against a candidate that they've been a decade in the same place, not widening their experience - I know people who are in precisely that bind, now they want to move but potential employers see them as stick-in-the-muds. Interviewers asking that question only want to hear an acceptable answer, implying that you've learned everything your current job had to give you, and you're ready to move on and up.