I know. Never claimed he was.
Trump could have ordered Sessions to release all the investigation documents on McCabe, and ordered Sessions to fire McCabe.
Comey, by the way, was another Obama holdover Trump failed to fire.
Trump knew the FBI had been snooping on him and his campaign. NSA Director Mike Rogers told him when he went to visit him at Trump Tower after the election. I think President Trump wanted to give Comey a bit of rope so he could hang himself...and he did.
Since McCabe was career FBI, I believe Sessions took the most logical route, by letting the IG do an independent investigation to look for wrong-doing, then letting the Office of Professional Responsibility (OPR) recommend he be fired. It was a disciplinary procedure handled through the normal channels. Had he simply been fired by Sessions without those steps, McCabe would have had grounds to sue for reinstatement. I'm not sure if there is any way for him to appeal his firing now since it was done according to the policies and procedures followed in disciplinary cases.
Having worked for a State correctional department myself, the only way you could be fired for work-related issues, was to be brought up on departmental charges, have a hearing in front of a (hopefully) independent mediator, and await the mediator's decision in the case. There was no option to appeal whatever decision he chose to issue. The State would follow through with whatever recommendation he/she made in the case.