Retainer mining: why does it have collectibles

[HyoMinPark]

I was confused for a second. I thought you'll get a collectable with an actuall collectability - Like 380 Torreya Log.

So when do you actually get this? I sent all my retainers either on quick exploration (for maximum effort) or for specific items (hunting, mining, etc).

You can't. If you want collectibles, you have to gather them yourself. Retainers cannot bring them to you.