There are no aliens at Area 51. The reason you can’t go onto the base is the same reason the Skunkworks tests advanced systems there: the place is radioactive and will be for hundreds of years.
In the 1950s and 1960s, the US Government tested nuclear weapons in and around Area 51. The infamous black and white video of a house being ripped apart by the shockwave from an atomic blast? That wasn’t far from Area 51. In fact, Area 51 gets its name because the region was broken up into areas for researching fallout.
The weird lights in the sky that have been seen since at least the 1970s? Aircraft, balloons, and drones (back then they were called “radio-controlled planes” and, yes, those existed and have existed for a long time) equipped with new technology. Those systems are tested there to make sure they can function in a radioactive environment.
Don’t believe me? Then do what I did. Take a relatively new car out there and drive as close to the property line as you can. Turn around and drive home. Then, try to wash your car. It’ll never come clean. The dust from the place literally burns its way into the clear coat (that’s the protective layer of plastic they put over the paint to keep the paint from flaking). You can breathe the stuff in, but if you’re exposed to it over long periods of time you’ll get exotic cancers and die from it.
There are no aliens at Area 51. There are no crashed spacecraft at Area 51. There’s just a lot of toxic dust the US military uses when testing new systems and weapons platforms to see if they’ll function in a post-nuclear war world.