Why are they actors in international relations?

Most classical IR theory (i.e., realism, liberalism and world systems theory) considers that the actors (main participants/decision makers) in the international sphere are the states. That means that decisions are considered as being aggregated at state level and that the international system is the creation of the states.

Constructivists and structuralists (as well as a few neorealists and neoliberals) add other actors, such as non-national entities (multi-national corporations, large NGOs), subnational entities (such as structures of government competing for ideological dominance, federal regions, lobby and pressure groups and even individuals in power) and transnational entities (such as international organizations in themselves as opposed to being just expression arenas for states) as well as ideatic structures (such as securitization complexes, social constructs, social norms and laws, ideologies etc.). In these schools, defining what an actor is is left to the author, so it is best to read carefully so that you can understand what the actor considers.