I'm curious about that as well. I can listen to an audiobook while I'm doing something else, like driving or folding laundry, and I feel like I retain it pretty well, but if I try to just sit down and listen to one, my mind will wander and I'll barely pay attention, much less retain anything. For non-fiction books, if it's something I have a really strong interest in, I can listen and retain. Like for instance, I listed to a book about Narcissim while going through a divorce. That held my attention really well as it got me thinking about a lot of things. However, I've tried some self-help or professinal development books, and I just can't pay attention or retain it.
I will say on the fiction, if it's a really complex book with a ton of characters (and those characters disappear for large portions at a time), I sometimes have difficulty. I just finished the Stormlight series and in the later books I was constantly confusing which character was which.