One reason is that the Apollo program, which sent humans to the moon, was a major achievement for NASA and the United States, but it was also very expensive. The cost of the Apollo program, in today's dollars, was approximately $100 billion. As a result, there has not been a strong political or financial motivation to return to the moon.
Another reason is that there have been other priorities for NASA and other space agencies. For example, the space shuttle program, which began in the 1980s, focused on building and servicing the International Space Station (ISS). The ISS is a collaborative project between several countries and is primarily used for research in microgravity. NASA and other space agencies have also focused on exploring other destinations, such as Mars, and developing new technologies that could be used for future space exploration.
Finally, there have also been technical challenges to returning to the moon. For example, the spacecraft and rocket technology used in the Apollo program is now outdated, and new systems would need to be developed. There is also the issue of developing new landing and surface exploration capabilities, as the equipment used in the Apollo missions is no longer functional.
Overall, while it is technically possible for humans to return to the moon, it would require a significant investment of time, money, and resources.
This is less than the 10% of the current investment USA is doing only on the Air Force. USA spends 1,5 trillion anually on the air force only, not including marines, etc.
Just going back for a quick jaunt to get more moon rocks can be done with robots.
The other game changer is the US isn't the only "we" working on landing humans on the moon.
We've had many robots on the moon since then. Yutu-2 has been roaming the surface for nearly 4 years. The Chandrayaan-3 rover is planned to land in summer.
But you mean people, I know.
Sending people to the moon is far more expensive than sending robots, which don't need life support and don't need to return to Earth.
Robotic probes gave us images of Pluto, and helicopters on Mars, and other amazing events. The Dragonfly mission hopes to have drone flying in Titan's atmosphere in 2034.
Of all the amazing things that could be done, why return people to the surface of the Moon when the same amount could do so much more amazing robotic exploration?
Artemis cost, what, $4.1 billion for a single launch, with an estimate of $93 billion project between 2012 and 2025? While all of Mars 2020 cost $3 billion, and New Horizons cost about $700 million.