If you can keep the supply low and slow you can make a lot of money on it.
In many cases I suspect it is not in the interests of the really big builders to make homes faster, as they can sit on the land they've bought and got planning permission for and watch the price of housing increase significantly faster than the costs involved in building them.
This is likely the truth. Land is expensive and they aren't making any more of it. They are far better off making more money per plot than potentially depressing the overall value of their houses by putting them up quickly in higher numbers. .