Because he wasn't a monarch and his political system wasn't monarchical. That's like asking why Stalin didn't call himself Tsar.
Politically, in his early days he also needed to rely on the support of conservative monarchists who saw him either as a pliable pawn or a lesser evil compared to the socialists, communists, and liberals. Their hope was to use the Nazis to maybe someday put a Hohenzollern back on the throne. If his ambition was to make himself the emperor, then he'd never get their support.
In 1934 Germany's head of state was a president not a monarch. When Reichspraesident (Reich President) Hindenberg died Hitler declined to take the position of Reichspraesident, claiming out of respect, and instead combined the positions of Reichskanzler (Reich Chancellor which he already held since 1933) and Reichspraesidnet into one position called "Fuehrer und Reichskanzler" (Leader and Reich Chancellor). This made him both head of government and head of state and this is when he technically became the dictator of Germany. Hitler worked within the system, even if he was technically breaking the laws of the system.
Edit - added about HoG and HoS.