They just don't want to. "Not how I always knew it!" is the catchall term to defend craven adherence to bronze-aged thoughts on biology and sociology. Yes, as we advance from age to age, we tend to gain better understandings about the world around us, and ourselves. Biology is, probably, the most fluid example of how little we knew then translates into a world of difference today.
I find that that those who are most uncomfortable with the strange things we learn in Biology simply don't understand how this works--how little we actually know. I mean, it's fine. But there is something about a physicist or engineer or chemist, whose disciplines are more or less immutable, that simply don't appreciate how much we learn, how quickly our understanding changes, from year to year, when it comes to Biology. It's just like science--only really much harder for lots of scientists. ....and stuboorn assholes that refuse to give a shit.
I mean, that's fine. Just admit you refuse to give a shit without demanding that you actually know what you are talking about when you so clearly don't.