It's a matter of perspective. Many of those still serving just see a continuation of what they know, the safe option, a good salary (most don't appreciate how little money they are actually working for - see post 5 on this thread) and don't believe their skills are appropriate outside for them to find gainful employment.
The vast majority of those who have left can't see the problem, don't look back, have a better life out than in, and are amazed in hindsight that they were at all reluctant to leave in the first place.
Those that have left the military, especially if its recently, can still remember their thoughts, feelings, concerns about leaving, and so can appreciate the position/dilemma of those still serving. By contrast those still serving can't really appreciate what it's like to be actually be out, as they have no experience of it.
At least that's my perspective....