When Pokémon Legends: Arceus was first announced, many people were surprised that the starters weren't new and original - nor were they the starters from the Sinnoh region of Diamond and Pearl. Soon afterward, it was revealed that this ancient Sinnoh region was called Hisui, which led to speculation for why these three Pokémon were chosen as Legends: Arceus' starters. However, there's actually a clear story explanation for why Rowlet, Cyndaquil, and Oshawott are the starter Pokémon for Hisui.
Once it was revealed that the game would be taking place in ancient times, fans theorized these starter Pokémon in Legends: Arceus shared a theme - their final evolutions were inspired by aspects of Japanese militaristic traditions. Oshawott's final evolution, Samurott, is inspired by the samurai. Decidueye, the final form of Rowlet, harkens back to the Japanese archery practice known as kyūdō. Meanwhile, Cyndaquil's final evolution, Typhlosion, has a different name in Japanese (Bakufoon) which could reference a form of the military government from Japan's history.
Thematically, these choices make sense. Hisui is inspired by ancient Japan (Sinnoh was based on the real-world island of Hokkaidō), so having Pokémon in Legends: Arceus that reflects this theme is logical. However, the game also provides a narrative reason for Cyndaquil, Rowlet, and Oshawott being the starters.
Pokémon Legends: Arceus - How Rowlet, Cyndaquil, & Oshawott Got To Hisui
At the beginning of Pokémon Legends: Arceus, the protagonist is sent back in time to the Hisui region. It is here that they first encounter Professor Laventon, a member of the Galaxy Expedition Team who is studying Pokémon and attempting to complete the first-ever Pokédex. He has Rowlet, Oshawott, and Cyndaquil in his care. And when the Pokémon run away, it's up to the protagonist to find each one during the tutorial before deciding which one they want to keep as a companion.
It's here that Professor Laventon reveals why Legends: Arceus' starters are these three Pokémon. He brought them from other regions and none are native to Hisui. This explains why the only others that can be found are in Space-Time Distortions. The original Sinnoh starters, though rare, can all be found in the wild in Hisui's various subregions. This is also used to explain why the starters' final evolutions are different in Hisui - the region caused their evolution to change. It's a pretty cool story tidbit that can get overlooked that early in Pokémon Legends: Arceus, and it does provide a canonical reason for why the game's starters aren't from Hisui.