Because the plan is not to just go to the Moon and back, but spend as much time there as possible. That may not always require going to the surface. It's also a lot easier to launch multiple landing missions from the gateway than from Earth since you can use it to pre-stage resources needed for landing.

I'm too lazy to look it up now, but the Lunar Gateway mission has been convered by many people on YouTube, so just look it up, and they'll explain it much better.