Why cant mankind repeat the moon landing if that ever happened in the 60's?
As stated, it was repeated 6 other times
after Armstrong and Aldrin.
Please note that there were 10 missions prior to Apollo 11 which prepared us for the final descent and landing, including two (IIRC) that went all the way there, orbited the moon and returned safely. All of this was done at great expense and with great care before we finally touched down.
Since then, there has been no reason to return humans to the moon. Because even as technology has advanced, there is still an 11% disaster rate for launching vehicles into space. You simply cannot open up a space flight industry to the public knowing that 1 in every 10 flights might blow up before ever leaving the atmosphere... So no, it doesn't follow that we should be vacationing on the Moon by now.
Also, every space agency in the world right now is focused on getting humans to Mars in the next 20 years. That is our next great endeavour and this time it's a global collective goal. We are no longer pressured by wars and political motivators - the goal has switched to that of being almost purely scientific, which is amazeballs. That's a real scientific word.
If we needed or wanted too, we could go back to the moon in just a couple of years, but that would require reallocating resources from all over NASA into one collective focus, and for what? Just to say that we can still do it? There's no real value in that. There's not too much more than we can learn from sending people to the moon... It's just not something we need to focus on - simple as that.
Want to see what's going to be taking us to Mars?
(This video was taken yesterday....)