well there's no denying that Area51 exists. That's been very public for many years.
Most of the higher res images are taken by airplane. My g/f does it, although what she does is rarely for Google Earth, more for farm associations. EVERY military base in the US has restricted airspace above and around them. Sometimes the airspace only goes to 10k feet, and other times it goes higher. Rarely does she fly above 8k feet. In order to fly into the restricted airspace, you have to ask permission first. Sometimes it's just on the radio when you're ready to enter the airspace. Other times you have to make arrangements ahead of time.
Now don't you think that if you have to make arrangements to fly over a high security area, that they'll have plenty of time to put away whatever they don't want people to see? Also, you can be dam sure that everybody that they are tracking everything that enters their airspace, and I'm not talking about watching you fly on a radar scope.
Because of the Nellis Air Range about half the state of Nevada is restricted airspace. Much of restricted air space has nothing to do with bases on the ground. Part of it is because of that area being used for missile and rocket tests. Part of it is because of ACM training. Part is because of other flight training by military personnel. There might not be an actual base within 100 miles of restricted airspace. There are _lots_ of reasons that airspace can be restricted and only a couple have anything to do with air bases or secret installations.
The Air Force still will not admit that Area 51 or the Groom Lake Facility exists. It is a black base and is not on the "public" books in the Federal budget. It is a secret facility even though people flying into and out of the base using chartered flights out of McCarran airport in Vegas is well documented.
Just because a base is a classified facility and is not acknowledged by the military doesn't mean that it isn't written about, photographed or filmed. It just means that the Air Force does not confirm or deny it's existence. They don't talk about it at all. This is in keeping with the National Security act for anything that is classified.
Area 51 is still _secret_ because no one outside its security halo knows what goes on there. Lots of people watch, film and speculate on it. But none of them knows what really goes on there or what that installation is for. It is rumored that the F117 was tested there. I have never seen official confirmation that that is true. Probably because that would mean that USAF would have to acknowledge that the base exists. And, to my knowledge, they have not.