‘Scientists’ - those involved in evolutionary biology, at least - don’t think that and never have. Evolution is known not to work by building things piece by piece. Almost all evolution consists of duplicating, modifying and combining existing ‘pieces’.
Nor is Irreducible Complexity (IC) an impossible concept for ‘science’ to grasp. It is grasped, it’s just not accepted. Nor should it be, not just because the way evolution actually works means that IC is not an obstacle, but also because there is evidence for IC systems evolving.
IC is a creationist concept that is only convincing to those who don’t know enough about evolution to understand how it happens in practice. Nor is it helped by the fact that the main examples of IC that have been touted by creationists and IDers - flagella, blood-clotting, eyes - are all demonstrably not irreducible.
P.S. Anyone who wants to see who really doesn’t understand IC should check out this thread.