The purpose of Shadow-Spawned as a separate card is so that it doesn't receive a resource the first time the Hunting Horror enters play. And since the resources on Shadow-Spawned are the only tokens that remain on cards that enter the Void, this allows you to place Doom on the Hunting Horror, which go away when it moves to the Void; but still keep resources. Having the Horror's card text say all of that would be an exceptionally wordy card.