I might be confused but i feel this is not as simple.
For example in my city there is many multi story houses get built all the time (averaging with 25 floors). As when i have opportunity to watch i never see empty house after it was finished. Usually it takes month or even less after all place cleans up before it lights all around and has all places for cars taken already.
Yes, if you want to buy flat you have huge lists of offers. But you almost never can just look and find new flats in houses built 1-2 years ago selling by developers. In any case this is speculators reselling what they bought previously.
And usually developers start to sell flats even when there is no house to show! Usually it happens when there is only hole in the ground for future house.
Personally i bought new house but i bought it from previous owner who bought it from developers and resold after 2 years for 1.5x price. It means 2 years after houses were built there was no one still for sale by developer.
i can see it wrong but for me it looks like demand exceeds supply by huge. This allows prices to rise, this allows speculators to resell, this allows developers to sell flats 1 year before house is ready.