It is because they've been taught it is from as far back as they can remember.
It is not found or even suggested at in Scripture, so there is no way that reading Scripture would lead to that conclusion.
The doctrine was developed and then the search for verses to support it ensued. Cheery pick the verses, take them out of context, and squeeze them in to fit the doctrine.
It's the only way that anything could even be found to remotely support the doctrine.
So?