Why do oligarchic governments often turn into tyrannical governments?

An oligarchy government is defined as the type of government in which all the power is centered to a few families, businesses, or individuals to rule and those rulers form the policies for the benefit of society.

While tyrannical governments is ruled by an absolute ruler, who uses his/her power for their benefit only.

The Oligarchy government often turn into tyrannical governments because in oligarchic governments, all the policies and legislation is controlled by the ruler only and uses their power for their benefit only without caring about society.