Simply due to North Korea. Despite all talks of normalization between South and North Korea, it's still not safe to let your guard down. Such talks of normalization took place few times in the past, going back as far as 1960s I believe.
Things like military installations being obvious from aerial imagery because it's the only areas where someone incompetent at using the Photoshop clone tool has copied a bunch of trees around.
Other examples are "secret" missile silos being discovered because a bunch of buildings are seen in the exact size / configuration as other known military sites. The South Koreans having re-used the plans and architectural drawings.
They said that in some cases the most plausible explanation for some sites still being secret is to save face. Nobody wants to own up to how much of an open secret they are due to opsec failures over the years, even though everyone, and certainly the North Koreans, know exactly where they are and what they're for.