You asked, "Why cant I use spotify with iphone 13 pro offline unless the music is downloaded."
Answer: This is because when your iPhone is Offline (No Wi-Fi and No Cellular) then the music has to be played from what has been already downloaded on the iPhone since the iPhone will not be able to stream over Wi-Fi or Cellular (Offline).
Now, if you actually meant to ask why Spotify can only stream over Wi-Fi and not over Cellular data, then I would recommend that you see whether Spotify app needs an update by checking in the App Store. If you are not using iOS 15.0.2, I recommend that you update your iPhone as well.
Axel F.