How Do Houses Prevent a Water Buildup in the Flooring?
How does this standard design allow this excess water to be shed by the house?


You are correct in seeing the flaw in a plastic vapor barrier. It doesn't allow the house to breathe. I personally don't like the idea.

The only way to shed the water is by evaporation through ventilation. Some of the water could drain to the ground since vapor barriers are not necessarily water tight but the tighter you make the house envelope the harder it gets to allow it to breathe.