You're making a lot of bold claims in this thread based on this 10% figure. But what's the evidence that builders are doing this as part of some sort of strategy to keep prices high and not a normal level of vacancy due to turnover and general market inefficiency? (Some assets in all markets will be overpriced and not sell immediately. How do we know that 10% is too high?)

Your theory also ignores the fact that builders have other ways of investing their money. Why build a house and let it sit vacant when you could have just not built that house and invested the money elsewhere?