If the governments told people extraterrestrials are on Earth it would likely destabilize societies and their economies. 

Due to their advanced technology and likely extensive knowledge of human history an exchange of information would likely disrupt religious views and cause massive panic, rejection of their ideas, and conflict. Imagine if everything you devoted your life to was wrong? Oil companies would lose their hold on the world economy and billions of jobs would disappear being replaced by better power sources.

Wars would break out and human societies would collapse. The survivors would have to reform cultures, economies, and religions to reflect the knowledge provided by the extraterrestrials

The world is not yet ready but due to a mass extinction event looming those in power may decide to release sooner rather than later. When mass extinction occurs the survivors would likely be more receptive to the idea of extraterrestrial saviors. 

Only time will tell the truth