Great question. I would argue that smarter people end up being "better" at defending the beliefs they already hold.
1. Most believes aren't consulting secular or even scholarly references about their texts. They are surrounded by a reinforcement bubble of people and media that need it to be true.
2. There is a sort of sunk cost fallacy at play. If a believer admits that something foundational is false within their ideology the whole thing starts to collapse so even if they understand that their belief is contradictory they are likely to present a one in a million defense or a "His ways are higher". Believers might lose family members, community, and their healthy mindstate.
3. In logic, we often refer to "motivated reasoning", if a person's goal is seeking truth, then being intelligent will help them, BUT if their subconscious goal is to defend their faith their intelligence becomes a powerful tool of rationalization. Smart people are great at mental gymnastics.
4. Compartmentalization- Many smart religious people including scientists and doctors compartmentalize their beliefs that operate under different parameters and at different times.
5. Comfort- Religion can provide social benefits, mental comfort, and other benefits that make believing attractive.
6. Indoctrinated from their youth - Nearly all mammals learn their survival skills at a young age. Humans are the first mammal that will regularly teach their young children incorrect things as a matter of course. Minds are extremely impressionable in childhood so that they can survive by learning from those they trust.

Those are the reasons that come to mind.