So, why did the real military surplus stores disappear? I'd think that
the military has just as much, if not more, aging items it could sell
off. What happened?

I'd guess that their customers disappeared. Shopping at
military-surplus stores just isn't what your typical American consumer
is into these days, just as we don't go to railroad-salvage stores
anymore to buy cases of dented cans of Campbell's soup. Just the same,
there's Sherper's and American Science and Surplus here in the Milwaukee
area that usually have plenty of real, honest-to-goodness military
surplus stuff.