Let’s try an analogy. Suppose you and your spouse own a small business and you suspect one of the employees is stealing. Who would you ask to investigate, your spouse or a private investigator? Your spouse who is 100% on your side, has an equal stake in the outcome, but that might be used to challenge their objectivity when they had to testify in court. On the other hand, a private investigator would be seen as unbiased, professional, and without a “dog in the fight”.
And that’s why Mueller did not fill his staff with pro-Trump investigators.
Especially if that private investigator was a member of the same private club as the thief.