Why don’t we visit the Moon any more?

There's not all that much on the moon. Even originally, a big driving force behind the manned landings was the political race against the Soviet Union. Without that, the massive cost would have been much harder to bear. Nowadays, if we wanted to go back we'd almost certainly send robotic probes. This is vastly cheaper, with few downsides. But even here, there aren't a lot of unanswered questions that are considered important, or anything undiscovered that we might expect to find. If we had unlimited money and resources we'd probably go back, but as soon as these things become a factor the moon gets quite boring as a target. It's more scientifically interesting to visit Mars, which is much more interesting from a human point of view, with its more complex conditions and possibility of water and even life. But again, unmanned vehicles are vastly cheaper and easier to run – human exploration would by inspiring and so on but is too difficult and expensive to consider in the near future.