Nobody is denying that many people find military service fulfilling. But certain roles have extremely limited talent pools. The odds that you'll find someone willing to take a position primarily for fulfillment when the starting salary for a contractor is double/triple/quadruple/quintuple what government offers, the public service role is immediately starting at a significant disadvantage.

Besides hiring talent, it carries through to career advancement and development (which plays heavily into personal fulfillment!) which on turn affects retention. If you're thinking of starting a family and settling down, being able to have more flexibility and significantly more money is a highly attractive option.