Why Would World Governments deny the existence of UFOs?

In order to prevent panic and the collapse of human civilization here on earth.

Some people could deal with the truth: maybe 25% to 30% but others would run amuck and kill themselves and others and become permanently or temporarily dysfunctional.

When Captain Cook went to Hawaii within that culture the average person was not allowed to acknowledge the existence of White people because they were considered to be "White Ghosts" of the dead. Only The Kahunas (medicine men and women) and Hawaiian Kings were allowed to communicate with the white Ghosts or to acknowledge their existence on pain of death. And when Captain Cook kidnapped the King he was killed for it.

So, in some ways it appears we are in the same cultural condition here on earth where aliens have to be denied even if we are related to many who visit because of the planet our ancestors blew up which is now the asteroid belt. (which was proved by a Soviet Probe there in the 1970s) but which was covered up by the religions of the world and kept out of the western press at that time.

So, it could easily be said that it is our religions that mostly prevent the acknowledgement of our relatives from space.

Because it is they who prevented the knowledge in the 1970s from spreading through the news (or anywhere in the western world) that the asteroid belt was nuked out of existence. Then one of those asteroids came here and conveniently killed all the big land dinosaurs so humans could live here right now.

But, here's my question: "If the Asteroid belt planet was nuked out of existence as a planet 65 million years ago already, how many technological civilizations have there been on earth like ours between ice ages since then?"