In some cases it allows the agency to operate freely regardless of the political environment and in others, there are historical reasons for the way they operate (ala NTSB and FAA).

It also allows the market to decide what risks justify the cost and which ones don't via the insurance market. These advisories are front and center in lawsuits when dealing with repeat occurrences so the pressure comes from economic forces. Thankfully, there are limitations to what the market is allowed to decide so this may eventually become a regulation through public pressure or agency review.