Why?

That's how all consultancies are.

Consultancies gives advice, but it's individuals who are giving that advice.

If the advice is really really wrong, you don't want liability to taint the rest of the organization.

For example, look at what happened to Arthur Andersen after Enron and WorldCom even though majority of the partners were not in on it.

It's the same story of accountancies, auditing, and law firms as well.