Why is the US also called Uncle Sam?

[pinoypower]

I am always wondering why the United States is sometimes called Uncle Sam by some writers? Why not Uncle Bill, Uncle George or Uncle Benjamin?

[patis]

Uncle Sam was named after a meat supplier during the War of 1812 (Americans vs. the British, http://en.wikipedia.org/wiki/War_of_1812). Uncle Sam's real name was Samuel Wilson, and he stamped U.S. on the packages. A somebody saw the stamps and jokingly suggested that the initials referred to Uncle Sam, and that's how it all began. (http://home.nycap.rr.com/content/us_bio.html)