I've worked in construction, and that's actually not the case at all. What happens is very similar to what happens in software development: someone realizes that the spec is borked; the subcontractor and general contractor get together to look it over, and come up with a kludge that fixes the problem as easily as possible.

If the change is at the customer's request, they get charged a nice premium and the necessary changes are made. At the end of the day, everyone still gets paid regardless of how often the customer changes their mind.

This, then, is perhaps what the trades do better than us: they're very upfront with the customer about added costs. "Yes, we can make your bathtub a jacuzzi. It'll cost you $2500 more and the job will take a couple days more. Do you want to go ahead with your change?" Because there is a clear and complete plan that the customer signs off on before construction begins, it's very easy to show them what a change in requirements entails.

Everyone can envision what knocking a wall down means, and can see it will cost money and time. Many decide, upon reflection, that they really don't care enough to pay that price. Others go nuts and end up with expensive projects that take forever. Either way, the contractors get paid.