Seniority
Recent posts have hit the nail on the head...where else would you go. Also the longer that you're in BA the closer you are to a training job, command, better pension, better lifestyle-everything that goes with seniority.
This is a career opportunity. I have no figures but I'd place a hefty bet that 1% or less leave voluntarily before (say) 60. After that...people want different things from life. If you really want to be a millionaire you shouldn't have gone flying in the first place anyway.
Moi? I'm sitting pretty chasing my second million-I gave up on the first!