It's vague on purpose, as vagueness gives the appointed bureaucrats more power in their rule-making processes. Strict, detailed laws don't give them any extra leeway to make rules that favor rent-seekers and friends.
That's one view. Another is that strict, detailed laws without human judgement inevitably creates absurd results.
Law doesn't work without room for judgement. Enforce rules designed for toy models of reality[1] and people won't respect the law, but rather fear the lash. Of course, a certain type of person likes that, but (presumably) those who want to live in democracies don't.
[1] I'd love to believe that someday, certain economists would figure this out, too.
On the flip-side flexible rules allow for regulatory agencies to adapt and respond to changing conditions without having to wait on a legislative body to get around to updating its rules.
No, it makes them into regulatory bodies. Would you really trust Congress to legislate the minute details of FDA clinical trials or EPA pollution regulations? How about transportation safety codes or allocation of NSF/NIH funding for basic research?
This is a fundamental feature of most modern democratic bureaucracies: the power of regulators flows directly from the legislature which sets the goals and scope of each agency but it is up to the individuals in each organization, who have spent most of their lives in the relevant fields gaining direct experience, to decide on the specific implementation of the legislators' vision.
