First of all you ask “Is America hiding aliens in Area 51?” No matter how I answer that question, let’s say I say “no” then you ask “Why are they keeping it a secret?” Well, if I said no to the first question the second question is absurd.
Before even asking the first question, however, I’d have to ask who do you mean by “America?” Does that mean Donald Trump? Former president Obama? Congress? Who exactly do you mean when you say “America” who is keeping this all secret from us? Does that mean you? Are you America? Who is America?
Finally, I’d like to point out that stars and planets are dozens, hundreds, thousands and even millions and billions of light years away. No one can travel that distance, aliens be damned.