Area 51 is a Base for final testing.
It is not where Alien-Tech is kept. Nor is it were Alien-tech is tested or implemented. You have to understand how many spy-satellites are focused on it every second of the day just to catch a glimpse of what out Military is about to put out.
Alien-tech, if there is some or our most Classified Projects are Created in some of the most populated places in the Nation around Populations. The perfect example of this is “Skunk Works”.
Spying on a place in a populated area is much more difficult as many people come and go, pass by, visit etc. It also affords for covert security and counter security operation to protect those secrets without raising the eyebrows of even the most aggressor nations spies.
The old saying of “if you want to hide something, hide it in plain sight” is not just a saying.