Because I need the Torterra to at least be seen so I can finish the local Pokedex.
There will be one in Victory Road so you can see it there. Torterra is used by a female Ace Trainer named Sydney on floor 2F.
You never battle your friend (Dawn) in D/P/Pt. You will only battle with her (I don't know when). Also, her starter never reaches the final evolution (I'm not sure).