Because the Bible tells me so.
But do not forget this one thing, dear friends: With the Lord a day is like a thousand years, and a thousand years are like a day. Peter 3:8
I always find it interesting that folks who claim the Bible is literal ignore those parts that disagree with their man-made beliefs, this one, IS means represents, and "not by faith alone"
KEY WORDS: WITH THE LORD... What is a day with man in this earthly life? You want to claim that also is a thousand years??? This isn't ignoring anything. It's paying attention to the context. Something, apparently, some people are unable to distinguish because their man-made dogmas get in the way.
I'll have to save this one as another example of inanity and the ridiculous measures some take to nitpick just to avoid admitting they were wrong.