It just comes down to the fact that reading is active and listening is passive. If you're reading a book, the book proceeds at precisely the pace of your understanding. If you have to read a sentence five times to understand it, then you do, and nothing provides any pressure on you because you're the one in control. If you're listening to an audiobook and you don't understand a sentence or lose focus or whatever, it just keeps on plowing ahead. And while it is possible to rewind and listen again, it just can't possibly deliver information as effectively as reading.