There are various reasons:
(Based on what my other colleague tell me when I asked them once, they were here (same company) for 15, 12 years)

1. Age- they thought that they have reached a certain age whereas they will be having a hard time to compete with younger ones. (If they will move to a different company)
2. Feeling of Security- they already felt secured in their position in the company. They dont want to start on the low/junior level again in a new company.
3. Environment- They already felt at ease with the current environment, and they dont want to start anew when it comes to dealing with new colleagues, bosses,management and the place itself.
4. Salary - they're thinking that they might be offered lower than what they are currently getting as there are a lot of people who are available for the same job. (Scared of competition I must say).
5.Loans- they still have some loans to the company that they need to pay. (credit card, car, salary.. etc..)

And lastly.... which is a big factor i think.

6. NOC/RELEASE - They will simply not be given this privilege!