Well, apparently they can. It is happening.
Perhaps it was determined somewhere down the line subjecting a Special Counsel to the same approval (and/or creation) process as US attorneys presented possible conflicts of interest.
A Special Counsel can investigate members of Congress, after all.
A Special Counsel can investigate members of Congress, after all.
But if Congress wanted to provide a "conflict of interest" exception to the requirement for Senate approval of all US Attorneys, they would need to put that exception in the statute.