Because existence of a trusted authority means that there's no need to shoulder the immense costs of full decentralization. The fact that you can call someone

Because existence of a trusted authority means that there's no need to shoulder the immense costs of full decentralization. The fact that you can call someone is the relevant point, not the details of who you call. Blockchain only makes sense to use if two parties (1) do not trust each other, (2) do not have some mutually trusted authority, and (3) have no set of N partially trusted authorities whose shared authority they would each trust. In any other circumstances, there exists a more efficient method to establish consensus. In the case of calling the police, it establishes that there exists a trusted authority, breaking condition (2).