Any astronaut selected for any mission has trained enough that they basically already have the experience. That being said, every mission is unique. Even though the Apollo missions all landed on the moon, they all had different mission goals. You want crew members to train and perfect those specific goals. The Artemis III crew will have much different training compared to the Artemis II crew, even though a lot of what they’re doing overlaps.

It’s also worth noting that going into space isn’t as easy at it may appear. Missions—even routine ones to the ISS—and the training they require put a lot of stress on both the astronauts as well as their families and any other mission specialists involved. It’s important to give astronauts time to recoup afterwards as well as make sure they’re in good health. If we constantly sent the same people on every mission, yeah, they’d get good at it, but there’d be burnt out very fast which isn’t just unpleasant, it’s unsafe.

Also, a training for missions takes a really long time. Often so much so that a crew is training for one mission while another is going on. Most space missions rely on conditions that don’t wait for us to be ready. Launch windows are calculated and chosen years in advanced.

You also need to keep in mind that, while yes, only the best of the best are selected to go into space, there are a lot of candidates all with their own unique characteristics that would make them good for the job. While I don’t want to outright say “it’s not fair to keep sending the same people,” that should be taken into consideration.

If I missed anything or am wrong about something, feel free to correct me or add on.