You might be right. As a counter-argument I can only offer my anecdata of being a contractor who has met quite a few people deep in the bowels of large enterprises who I would class as brilliant, but who are coming up on five, ten and sometimes fifteen year anniversaries. Perhaps they do have good reasons for staying, but whatever they are those reasons are they are out of reach of my make-as-much-money-as-fast-as-you-can monkey brain :)