Because the alternative would be Russia or China, economically China, and such a shift in power balance would create incentives for a bigger war. The power assymetry of US vs NATO+China+Russia+Anglosphere, with US all alone, would create the power assimetry necessary to make a large scale war in big demand. Would that be a good thing?
Plus read some Thomas Hobbes. US acts a Leviathan, if US was out of the game it would just create a power vacuum to be replaced by another entity who would do equal or worse. You would just get invaded by Russia or colonized by China, who knows?
US simply filled the power vacuum of British Empire and Europe (WWI plus WWII destroyed Europe as a hegemon). If an hegemon disappears, another hegemon will replace it. That's how history goes. The disappearence of the current hegemon does not imply a world without the concept of hegemony.
It's a more abstract situation.