Why do chemtrail's or vapor trails only appear on commercial flights?

Why do you not see a chemtrail or vapor trail from the military jet?

   I could be wrong but living next to a military base I see aircraft in the air all the time. But the question that keeps coming to mind is why do I only seem to see vapor trails coming from high-altitude commercial aircraft and the only time I see chemical or vapor trails coming from military aircraft is during an air show? The majority of the time when a military aircraft is flying overhead I have to strain my eyes in order to spot the aircraft. No chemtrail, no vapor trail and that would prompt the logical answer that military aircraft do not want to be seen. But the basic question that I have is why do these other aircraft have to be seen either? Why can't they fly without a vapor trail as well?