> Why? The book itself gives no reasoning or exact age for when Earth started, yet we have to believe it is fact that someone misreading that book has the exact pure date?

Young earth creationism is predicated on the idea that every word in the Bible is literal fact (no exceptions), so you can “calculate” the age of the earth by the genealogy accounts of ancestry from Adam to Jesus. It falls apart at any reasonable scrutiny, but like you said, it’s not rational thinking. It’s dogma for a certain group of people.