These are appointees. They serve ‘at the pleasure of the President’ - though they are a sort of hybrid since they have a tradition of being much less connected to politics than other appointees, or did before Clinton did a mass purge and then had his cronies be part of the chain reporting to him about the progress of cases against him and his. The Dems just try to turn it into a scandal any time the Republicans fire any.
You could here nothing but crickets from the Press when Clinton fired/accepted the resignations of every attorney. When Bush 2 took office he replaced less than a dozen (I think) and the Press and ComDems spun up to High Order.