What would be the point in the government lying about an alien encounter?
What's the point about most of the stuff the government lies about?
The government lies as a matter of policy. Politicians play statistics to make themselves look better. Motivations for passing various laws are usually flat out lies. Beuracracies lie to perepetuate themselves or make their little world seem more important. It goes on and on.
The government also loves secrecy. People can hear a plane, see a plane, even take pictures of a plane and the government will outright deny that it exists if the program is somehow "secret." And they happily stamp "secret" all over trivial shit. It fits neatly into the ability to lie and deny, which is what the government does best!