Some categories are blocked, like daycare and schools. They'll also prevent anything from showing up, pokemon included, from areas that don't have recognized roads or paths. There is a park near my house that has a paved path through the woods that doesn't show up in the pokemon go app, so nothing appears there because the game doesn't want to encourage you to wander through the woods.

the locations are identical to the ones users submitted for ingress, the previous game niantic did.

for that you didn't need much vetting and because the popularity was low nobody cared so much. most likely some servicemen (servicepersons?) submitted the spots.

No, they're (mostly) user-submitted and user-approved. In theory new locations on military bases aren't allowed as of a couple of years ago, but a few slip through the process. And, inconsistently, old ones aren't removed.

They were seeded from locations used in a previous AR game called Ingress, however since (iirc) Ingress locations were user-submitted it lead to PokeGo’s landmarks sometimes being odd or awkwards.