Because there was no United States in the first century when John received the Revelation.
Do you think it will take 200,000,000 million troops to invade the country called Israel (Rev 9:16)? Israel has 7.5 million people. Who do you suppose they're invading?
No kidding.
I wouldn't expect John to say "The United States of America", but I don't think hoping for an oblique reference is too much to ask.
He could have referred to the land of the Eagle, a land not yet known, a nation of immigrants, the new Babel, or any of dozens of phrases that wouldn't be out of place in a first century text, but would grow in meaning to unambiguously (or even, throw me a bone here, ambiguously) refer to us.
We ain't there.
Therefore in the end times, we are irrelevant.
Or The Book is wrong.
Rev 9:16 says 200,000,000, not 200,000,000 million.
Given God and Israel's track record I don't think I'd try it with less than a 25:1 advantage either...