Because at the time it was written the authors weren't even aware of the existence of the continent it was on much less the country?
The Bible is inspired by God and He knew of the existence of America and that she would not be a factor during the Tribulation. By itself, the explosion of the Yellowstone Caldera followed by a 12.0 earthquake along the New Madrid fault would destroy America.
Well....that really tells us what you think about the Bible, its inspiration...and the ability of God to see eternally.
And Daniel made numerous prophecies about countries who had not yet risen to significance...namely Rome and Greece.