Corruption is one issue, but as I understand it there are also technical issues. The synthesis of derivative investments like collateralized debt obligations involves modelling the probabilities of default. If the models are wrong, such as assuming probabilistic events are not correlated (e.g. that mortgage default is a microeconomic phenomenon of individual hardship, rather than macroeconomic recession), then the ratings will not be sensible. Robust ratings require economic science to be robust.