I think that's because in drama classes they teach the concept of radio voice, which tends to be based on that, so they're like deliberately doing the performance in that accent, here in México there's a very similar thing where on TV people tend to use an accent that isn't particularly accurate to any region of the country in particular, you know because of like the radio voice, which is supposed to be like a voice that's more universally understood than a regional accent or something.
Edit: I just notice how much I repeat myself, LOL I suck at writing.
Well it's also since the narrator of the audiobook might have to have over a dozen distinct voices, they will tend to exaggerate some aspect of each character, so that the reader can easily differentiate who is speaking. This is also something they do when they are narrating someone of the opposite gender. That's usually my biggest metric for a good audiobook narrator, is how well they do the voices for the opposite. As that's usually where they break me. Somebody doing a really deep southern drawl for Joe American, while annoying, I mean, it's not like I don't hear people actually talk like that every fucking day, so I can't really say it's inaccurate. But someone with a really deep, gravely voice, trying to say, voice a 10 year old girl. Or a woman with a higher range of voice, trying to do Gruffy McGruffFace, the 75 year old mentor who sounds like....shit that singer who was famous for having a really gravely voice. Smoke and drank a lot, was a blues/jazz singer....fuck what was his name? Well, anyway, that guy.
TOM WAITS!! That's his name! But yeah, a woman with a high voice, trying to sound like Tom Waits, or Tom Waits, trying to sound like Shirley Temple. THAT is where I'm like "yeeeeeah, I think I'm done with this book."
Accents though, eh, I see the reason for them. It's basically audo short hand for the listener. Because not all authors do the "Jessie said" "Billy said" as regularly as others. So it will sometimes just be dialogue, then a switch, then back, without any written indicator of who said it. So listening to that, makes it difficult. But the voices, that's how you can tell "Oh, Jessie said that, and then Billy replied."