I liked the John Byrne era explanation, that their physiology was altered so that they would grow sick and die if they ever left Krypton because of their extremist isolationism, and Clark was born without that alteration.
"Actually, there was a justification for that one - AI Jor-El told her how to configure Kal-El's birth ship to recreate the Phantom Zone. That's why she and the scientist are working on the ship in the cargo hold. The plot contrivance is why Faora walks up to her at Kal-El's surrender and says "Zod wants this one too." I don't think it's ever explained why Zod would have the foggiest notion who Lois is, much less want her to come along. But she has to be there to have AI Jor-El tell her how to create the Phantom Zone."
If I recall the movie correctly, the Kryptonians observed her with Clark beforehand. So they took her as a hostage, to threaten violence to in order to force his cooperation.