Why can't they land on the Moon?

The premise is false: it is possible to land on the Moon, and it has been done with a crew on board. Apollo 11 lifted off on July 16, 1969, at 13:32 UT atop a Saturn V SA-504, and its lunar module touched down in the Mare Tranquillitatis on July 20 at 20:17:40 UT, after a 756.3-second burn of the descent engine. The crew spent 21 hours and 36 minutes on the lunar surface and lifted off again at 17:54:01 UT on July 21.

What is true is that a Moon landing is more demanding than a return to Earth. The Moon has no usable atmosphere, only an exosphere so thin that it isn't breathable, doesn't cushion impacts, and offers no protection from radiation. That removes at a stroke the two resources that make an Earth reentry cheaper: the heat shield that converts speed into heat, and parachutes. On the Moon, every meter per second of speed that must be shed has to be paid for with propellant, firing the engines against the motion, and the vehicle must also carry the fuel needed to lift off again.

The current program reflects that difficulty. The Moon landing doesn't depend on the Orion spacecraft, which only reaches lunar orbit, but on a separate human landing system that is tested beforehand on its own. Under the current schedule (as of October 2026), Artemis's first Moon landing is Artemis IV, planned for early 2028: two crew members will descend to the surface and spend close to a week in the lunar south pole region, an area where NASA identified nine candidate landing regions in 2024, then intended for Artemis III.