Earlier quoted context omitted.
For the same reason the US considers its self the "leader" of the world. You can't have it both ways, if you want to "lead", you've gotta pay for it.
Has the U.S. ever claimed to be the leader of the world? I do not think any country has ever claimed that title. The closest to it would be some of the remarks that are supposedly made to each Pope upon becoming Pope. That said, there is the similar sounding title called “the leader of the free world” applied to the U.S. president since the end of WWII. I always thought that was the result of military alliances, not…
This means they have the capability to enforce their will globally to a significant extent. In an arena such as geopolitics, justice is the will of the stronger, no holds barred. This makes the USA arguably the primary concern in geopolitics, the ring you need to kiss to do anything on that stage.
Keep in mind that “claimed” may be referring to the sense of “won” rather than “stated”.
From that perspective it’s not too much of a stretch to call them the world leader, but that does ignore the fact that leadership implies the will to lead and to a significant extent the requisite wisdom and skills.