Creationists HAVE to have a young earth or else it breaks their religion (original sin and atonement).
More accurately, they don't understand their own holy books, their cultural context, and history. A lot of them think their holy book was written all at once, and was always a single collection, instead of the reality of their being assorted writings and stories collected together hundreds of years after the fact and canonized.
They see "six days" and take it literally, without any interest or understanding in the fact that it is a translation of a translation with centuries old grammar and idioms. Some concepts don't exist from one language to the next, and so on...
Even in English, the word "day" has multiple meanings, and they've chosen one definition and made their narrative to fit a "day" as a 24-hour period... rather than the second common definition.
So I don't blame creationists for having to believe in creationism for their religion to be valid.
The problem is one of the recurring themes of humanity: Somebody (politician, theologian, philosopher, artist) says "this is reality", without any citations/reasons/facts, and masses follow, because it's easier than thinking. It's not really a fault of religion, because we see the same behavior independent of religion.