Why does Ryze's ult not warp out of Mordekaiser's ult?

This is purely a gameplay/flavor question. Both ultimate have "realm" in their skill name. So I was wondering if it would make sense that Ryze can warp out of Mordekaiser's ult. This would imply that Ryze can warp across realms which might expand the league universe too.

On a more technical and balancing aspect, I understand there might be issues with some champions having the innate ability to cancel someone else's ultimate. But it would could if you could do that and Ryze would just warp back into the same spot on the normal map.