For the same reason that people accept religious authority. Starting at a very early age in their lives, they are indoctrinated by threats of eternal damnation or by compulsory education and forced to idolize and defend these illegitimate authority figures who have an interest in perpetuating this cycle of threats and force.
Legitimate political power is recognized when the majority of people accept that leadership. In the case of the US Constitution, people made the decision give the document legitimacy by voting for it.
A republic is a system in which ultimate political authority is vested in the people.
People
SOVEREIGNITY
Citizens or sheep depending on which side of the government authority you are on.
Parliament's authority
popular sovereignty
political power spectrum
The idea that the political authority of government comes from the people.
The ultimate authority lies with the people. In Islam, the ultimate authority lies with Almighty Allah.
political power spectrum
Traditional authority,legal authority,and autocratic authority.