Always saying "United States of America" would be rather cumbersome.
But it's the reward for being the first country on the content to become independent. Everybody else afterwards had to pick more specific names tot avoid any confusion.
BTW Columbia was also frequently used as a generic name for the American Continent back in the 1700s and 1800s. The modern country of Colombia co-opted it in a very similar way (well originally "Gran Colombia" was supposed to include entire Hispanic America it just didn't work out that well...)