Because it was a space race during cold war between US and USSR to see who could reach the moon first to raise one country's pride. If moon landing was fake, USSR would be the first one to dispute it. Why didn't USSR do that? Because USSR knew that it is real and the space race to the moon was over.

Why no one going there since then? The technology was there but just "too expensive" to send human beings to moon.

Now why people interested to try to go to moon again? It is because the space race flame is lit again between US and China to go to Mars. Because Moon is a stepping stone to go to Mars.

Indeed.

Why didn't we hear any reputable scientists from USSR and China to dispute the US moon landing?

Instead we have those foolish conspiracy theorists and flat earthers to dispute that.

If I am not mistaken, the laser reflector on the moon doesn't need any power to run. It is just a array of prisms, like a complex mirrors, to reflect light.

Anybody on earth with enough power laser could shot a laser beam on that reflector and would get a reflection back from the moon.