The Department of Justice is not the president’s personal legal team, designed to lock up his rivals. DOJ has a long tradition of independence from the White House. Bowing to the wishes of the president to investigate his political enemies would undermine public confidence in the objectivity of DOJ’s charging decisions in this case and all others.
It may be, as some have speculated, that the new investigation is designed merely to appease Trump, and that officials know full well that no charges will emerge. Even that sort of charade would be an abuse of the awesome powers of the Department of Justice and a waste of resources that could be better spent on new cases.