What's in there?
Maybe it's actually a secret resort for the high-ups in the millitary-industrial complex, and they don't want anyone to know they have a billion-dollar resort up there?
Maybe the aliens hang out there, too?
Sure, of course it's a testing ground for new high tech stuff. They need places for that. But I mean come on, if I saw a new spy plane, I'd be no security threat because I could describe the thing as being no better than "it's a plane. Duh."
Maybe they have remote controlled robot birds with cameras for heads that they don't want anyone to know about. Even if our enemies knew about them, what are they going to do - hire paramillitary bird watchers?
Well, I guess that's so unbelievable to the non-tech countries, that we could make realistic robot birds with micro-cams, that we should keep that secret if we could get away with that.
But what's really in AREA 51? Elvis?