You’re mostly correct in this case, but for more commoditized houses or complexes, that can generate income, the reasoning is as follows:

There’s always significant uncertainty in construction: timelines, permits, cost overruns, contractor vetting, etc. In addition, two years you spend building are two years of rents that you don’t collect. As a result, there’s usually a premium, sometimes a significant one, for having something already built.

The amount of the premium varies depending on how those factors interrelate in a given market, and of course speculative appetite in that market also comes into play.