As for Area 51, it's mostly empty desert. It's the base in the middle of it that's where something is. Even if the USA government did announce that everything was moved out of it in the 1980s, that doesn't mean it was. It's a secret military base, so of course the authorities would lie about it if doing so served their purpose. They officially denied it existed for ages, even though everyone knew it was there.
Indeed, it would be a rather expensive distraction too that case, I don't think there is any need tbh... if you want to test new aircraft technology then having an isolated and well protected airfield that people can't get to seems like a good idea. That people know there is such an airfield doesn't really matter too much, it isn't like they can get particularly close to it and there isn't much official acknowledgement of what goes on there aside from some documents the CIA released about the history of the U2 spy plane and later Oxcart/the Lockheed A12. I mean it is pretty apparent, from what has been officially and unofficially revealed, that the US government tests new aircraft technology there.
Likewise everyone knows where the SIS(MI6) HQ is in London etc.. it doesn't particularly matter as you still don't know what is going on inside it at the moment. I mean they could keep it's location secret but foreign intelligence agencies would find out where it was fairly quickly and so there isn't much point.
Likewise the Russians etc.. likely know the various locations the US tests aircraft so it would seem rather pointless to move an expensive aircraft testing facility just so some UFO fans don't know where it is anymore.