audiobooks and reading books both activate the same language related areas of the brain

This doesn’t mean they’re the same thing. This is an area of ongoing research (because audiobooks have only recently become very popular) so there are surprisingly few studies on the topic, but the general consensus is that they’re not the same thing. For example, while reading you go at your own pace and can easily re-read or skim words or sentences, but you can’t do this when listening to audiobooks.

I’d link you to a nice essay I read(!) on this last year in a Finnish newspaper, but it’s in Finnish so most users here probably won’t get much out of it… Actually what the hell, I’ll link it anyway: www.hs.fi/kulttuuri/art-2000011260022.html