They don't have to make it up. They have 'experts' that they can refer to. Unfortunately, the experts seem to have cherry picked facts that support them. They accept the interpretation the experts promote.
Another problem that they have is they accept the interpretation that was decided on centuries ago. They don't examine the Bible carefully either.
I am a 'creationist' but I have no problem with C14 etc dating techniques, in fact I love them. They don't want to consider whether the Science is right. Many of you don't want to consider whether the Bible is right. It is easy to beat a straw lion. Happy hunting.