I don't think it is just Cosmology, though. There's a similar conceptual foundation to the idea of Subjective Reality and the idea of creating your own pocket dimension with a custom reality. The two ideas likely developed side by side with each one reinforcing the other, so it makes sense that we'd see Horizon Realms pop up much, much more in the lead up to the Ascension War.

Likely there were Horizon Realms before, some of them seem to be built on the remains of other things after all, but they weren't understood to be Horizon Realms until later....