I know that they're are rotten turnips but when people say "collectibles collecting dust" is that just like a saying that doesn't mean much or does something happen to them? This might be a bad question so sorry in advance.
"Collecting dust" is just an expression that means something is not being used. So if you own a collectible, but never display it, it might be said that it's collecting dust.
It's just a term for a collectible in your inventory not being used. For instance, if you had say, an apple collectible that you haven't used in your lineup in a long time, then you would say that apple collectible is "collecting dust", as you aren't using it or haven't used it in awhile.