Here in the real world, mankind is on the verge of leaving Earth and settling on other planets for the first time. We've been to the moon and plan to go to Mars. Baby steps in colonizing the universe.
In Suzerain, though, the Pulse War got in the way of our 21st century ambitions. Mankind very nearly died out, and it was centuries before we finally got a colony ship off Earth.
The moment those first colonists engaged the thrusters and left Earth behind, that's when the Catalyst realms begin. All our far-futures lie within Catalyst, just as our near-future is the Elements family of realms.