Parapets on the Roof

In last week's parsha Ki Teitze, the Torah set forth a simply safety law. "When you build a new house, you shall make a parapet for your roof, so that you will not bring bloodguilt on your house if anyone falls from it." (Deut 22:8.) The general principal is obvious to everyone, and certainly to any lawyer: take reasonable precautions to prevent foreseeable harm.

Why don't people literally do this? That is, why don't Jews put a small parapet around their roof to fulfill the literal wording of this mitvah? I've never seen one, at least for these purposes.

Under the literal wording of the rule, the obligation applies only to a person building a new house, not occupying an existing house. But certainly the general principal applies.