I think STS wants to avoid having people gamble with their gold. Back in seasons 1 & 2, before locked crates existed, I can remember some players spending all of their gold on elite golden chests only to strike out and not loot the best items, and then they quit the game in rage because they had nothing left. Spending your plat in this manor does not produce the same result since you still have all your items and gold and there is still motivation to try even if you did not loot a good item.

When you spend plat, you are gambling your real life money and not your money in the game. When you gamble your gold in the game, you run the risk of getting angry and quitting.