FBI directors are expected to conduct politically independent investigations.
While it's true that the attorney general is part of Trump's Cabinet, the president's remarks that the FBI director answers directly to him ignores
a longstanding precedent of distance between the executive branch and an independent FBI.
"As a matter of constitutional hierarchy, this is of course true. But in investigative matters, the FBI director does not, or should not, serve the president by reporting to him,"
wrote Benjamin Wittes, editor in chief of Lawfare blog and a senior fellow in governance studies at the Brookings Institution.
"He serves the president by leading law enforcement in an independent and apolitical fashion. And it is fundamentally corrupt for any president to be asking him to do otherwise... The astonishing implication of Trump’s view is that he believes the president may shut down an FBI investigation that displeases him."
FBI directors serve a lengthy 10-year term designed to guard FBI directors' political independence — and presidents typically do not want to give even the appearance of influencing the nonpolitical law enforcement agency's investigations.
For instance, while President Obama appointed Comey to head the agency, Comey said he only spoke with Obama twice in three years: once about law enforcement policy issues and once to say goodbye. By contrast, Comey said he and President Trump spoke nine times over the course of four months.
https://www.usatoday.com/story/news/politics/onpolitics/2017/07/20/fbi-director-reports-justice-department-not-president/495094001/