Because its a fantasy novel with magic and prophecies and a good fantasy author doesn't waste pages with useless descriptions of things that are largely irrelevant to the larger plot.
Oh.
Is he gonna keep some in reserve and wait to sell them at a higher price after they've been water damaged?
1. It's called misdirection.
2. That still doesn't mean every dream is a prophecy. Typically they are used to convey the emotional state of a character. Do you really think that Tyrion will end up riding a dragon fighting beside Bittersteel and have two heads and during a battle he'll slay Jaimie and Tywin? No. The point of the dream was to convey Tyrion's emotional state, his love and hatred for his brother, and his drive to get vengeance.