Most likely because they confuse faith with science and have trouble reconciling the two.  Science, by definition, makes statements that can be tested.  Faith doesn't require proof.  As a scientist, I can look at the evidence that supports evolution and recognize that it makes sense and has yet to be disproved.  I can look at the idea of intelligent design and say "That's not science, because it can't be tested.  Belief in that idea is based on faith, not scientific proof."  This does not, however, interfere with my belief in a higher power.  Science tells us how we got here, Faith tells us what we're supposed to do now that we're here.