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 For example, the spacecraft and rocket technology used in the Apollo program is now outdated, and new systems would need to be developed.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.