That's one take from one school of thought.
The millennium of Revelation 20 is not a future event. It was when Jeremiah prophesied, obviously, but is not anymore. Or, to be more exacting, it's no longer merely a future event.
Then why do so many believers think of it as yet future only?
For at least two reasons, I think:
1. A basic misunderstanding of Revelation as a whole, and the Millennium of chapter 20 included.
2. Many believers (primarily western believers) have bought into the heresy of the "rapture," which came about in the early 19th century. It's not that they are heretics, it's just that that's all they've ever been exposed to.
Your take may be similar or be completely different.
Please share it here, and tell us why you hold that position.