Why are princes Charles, Harry, and William always dressed in military garb even though they're obviously not on active duty?

There are occasions where I could wear my military garb if I chose to. I gave them away, though, as they no longer fit.

 

  I don’t know the United Kingdom’s rules and regs on the matter, especially taking into consideration they’re members of royalty, so my answer is based on my status as a veteran of the US Armed Forces. 

  In the US, there are certain conditions and situations under which the military uniform can be worn by veterans and retirees. One cannot, however, pose as or attempt pose as an active duty servicemember, nor exact/exert any military authority. 
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They were all in the military.  They have the right to wear their military clothing if they want.  Many former military people wear them here as well. 

They likely see it as a way of reminding people that the served.  And perhaps as a form of respect for the military. 

They are symbolic leaders of designated Units as well as Organizations

Harry lost the right except when at a Dining in to wear the Uniform when he stepped down.  He did serve in an active unit on deployment however so he might push Remembrance Day.

Served on a number of Joint Mission and learned enough to become very confused

They wear military uniforms only when appropriate to the formal occasion involved. The rest of the time they wear ordinary suits at work, and casual clothes when not. 

Formal reasons. All are former servicemen, as is their matriarch, Her Majesty Queen Elizabeth (served as an ATS mechanic in World War II). Ergo, they are not only entitled but required to be in uniform when the occasion demands it.