President Trump can ask him to leave at any time. For whatever reason, and the imagination can run wild, Trump has elected to keep him on. I think Comey has the goods on Obama, Lynch, the Clintons, et al. If so, firing him will not facilitate his cooperation in any current or future investigations. Keep your friends close and your enemies closer.
Following your line of thinking, he should make Hillary Clinton his Chief of Staff. Comey is owned and controlled lock,stock, and barrel head by Hillary Clinton.