When I delete the game and download it again. Why can't I find my old friends?

When you uninstall an app, you're gonna lose the data, including multiplayer status. Unless you made an account and uploaded data to it, I'm afraid it's lost.

You need to make an account, or else all is lost and you need to restart. Trust me, I have had the same issue. Hope that this helped

When you uninstall the game the data is lost and the info would be gone.

If they are friends offline you can Create a game and they can join it, then I would recommend to friend them so you can find them whenever you both are playing.