Many people do not understand that we have a ruthless up-or-out promotion system for military officers. Percentages are given below. So, 95% of lieutenants make captain and the other 5% get fired. That doesn't seem so bad but if you do the math on a lieutenants probability of making full colonel, it's .95 x .8 x .7 x .5 = .266 or 27%.
This goes on continuously, year after year for decades. Federal civilians don't have that risk, nor do enlisted troops. If a civilian is a GS-9, he or she can stay at that grade for 30 years and retire.