The Justice Department’s lawsuit against Standard & Poor’s this week has revived focus on the role of the rating agencies in causing the financial crisis. Yet four years after shoddy mortgage investments bearing top grades nearly took down the country’s economy, government attempts to revamp the rating-agency industry have stalled.

Nearly everyone agrees there needs to be an overhaul of the way complex financial products get rated for their riskiness. But attempts to change the system keep running into the same roadblock: The rating agencies, however flawed, have become so crucial to the workings of Wall Street that no one can live without them.