They haven't solved traveling through the Van Allen belt or softening the landing on the return flight, which requires carrying so much fuel that the initial takeoff becomes unfeasible. In the attempts to visit Mars, you can see them ask for help with passing through the Van Allen belt. Why can't they use the solutions from the moon landing? Because it didn't happen.

When the astronauts came back from their supposed trip to the moon, they couldn't even remember if they saw stars in the sky even though they must've seen them everywhere. There are countless indications that the footage of the moon landing was faked.