The DOJ employs lots of ‘outside’ people in the various ‘task forces’ over the years. Their use is under the same kind of regulations that authorize a SC. Those regs would be VERY tough to challenge, and a successful challenge would have much further repercussions.
The special counsel regulation is unique in that it purports to appoint a non-government employee to a newly-created position with all of the powers of a US Attorney (a new de facto US Attorney slot, except that Congress has not authorized the AG to create new US Attorneys) and at the same time makes it illegal for the DOJ to exercise day-to-day supervision over such person.