If credit agencies' business is just to serve as proxies of your credit worthiness, why is regulation of them necessary at all, since they'll always be consumer friendly?
That's a pretty weak argument. The existence of regulation - even heavy regulation - in an industry is not a barometer of their value to consumers.
But there are a lot of reasons why an industry who scores credit worthiness would be highly regulated, even if nothing had ever gone wrong (and plenty has). The main reason would be to ensure that score is applied in the same way to everyone, and that it isn't crafted in a way to create extreme disparities based on things like sex, race, religion, etc.
Or maybe there's more to it than that?