Because of their premise, those people (namely YECs) are constantly questioning scientific evidence- which is good- but then they take it too far by asserting that it is impossible for those evolutionists/other scientists to come to a correct conclusion.
The problem is that they don't see the inconsistency in questioning everything about science, but never asking a single question about their own faith. Questioning is good, as long as you do it across all beliefs, and have a standard level of confirmation you will adhere to.