Well, truth be told, it wasn't like no one knew the Werdlem Empire wasn't there. We must not forget that the twins' group heard several times that the Werdlem Empire received 'visitors' quite a few times over the years. Obviously, some powers knew about it.
That said, the real question was instead why no one found Lisan Universe before. After all, it was so close to the Werdlem Empire... or was it?
Well, you have to look into the context to see the full picture. The connection created by using some of The Echo in the Werdlem Treasures was much, much faster than any other method if you don't consider things like the twins' Celestial Soul Dimensional Realm cheat.
