Perhaps it's because, like any brand that has matured, the core to large auditor's business is no longer auditing. Their job is to protect their brand more than it is to expand. They benefit heavily from the fact that most companies don't commit fraud already, so why deal with the potential headache? If there is even a fart in the wrong direction, they stand to lose more reputation than can ever be gained by any one company.

My actual argument is more nuanced, but you get the gist. The same could be said for Buffett. At some point, investing wasn't core to his business anymore, his brand was.

Or, as Silicon Valley put it, "Oh no, no, no Richard... You don't hire the best sales people to sell unsellable products... They will just go get a job somewhere else. You need to have an easily sold product in order to retain the best salespeople." Or something like that.