Because he basically is. Every American presidential election is closely followed by other countries throughout the world. When Biden won in 2020, other countries were celebrating. When there is an international crisis going on, countries look to the United States for leadership. America has military bases all over the world, the largest and best navy in the world and has had the ability to fight a major war on two fronts since the 1940s.
The American dollar has been the reserve currency of the world for over 60 years.
The president is the commander and chief of the armed forces and the leader of the country. By default he's the most powerful man in the world.