Why go to science fiction, when there are real-world examples today, why humans don't want to go to the Moon or Mars for extended times?

Bone strength.

The alien's home is a 7g planet. They can live on 1g Earth for a while, but over time their bones get too weak, their blood vessels expand, and all the other ailments that humans have to recover from when they were too long on the ISS.

No, it is not an option for them to walk around with [16tons][1] weights as their Earth-shoes to compensate :-)


  [1]: https://duckduckgo.com/?q=16%20tons%20weight&t=ffnt&iax=images&ia=images