Reasons for staying in:
1. It suits me from a purely selfish point of view - good pay (PAS), good pension (PAS again), boarding school allowance.
2. I live in my own house with a reasonable commute & a number of stations around to potentially serve at for future postings.
3. I enjoy my job, & I hate route flying which would become my bread & butter if I left & joined an airline.
4. If anything changed any of the above, I'm off!