Why Is Direct Command by the President Over the Executive

Why is direct command by the president over the executive branch seldom feasible?

A) There is so much going on that presidents can't keep personal track of each one of the millions of government officials and employees.
B) Presidents don't have the power to hire and fire top executive officials.
C) Presidents have difficulty communicating their desires to top executive officials.
D) The president's directives are many times overshadowed by those of Congress.
E) All members of the executive branch, other than the president, are protected by civil service laws.