"How I Was Brought Up"

I was talking to a friend, a few weeks ago and I asked her why she felt a certain way about something (I can't remember what now) and she said "that's just the way I was brought up." I've been thinking about that answer for a a long time, and I still can't really wrap my head around it. Why would someone choose to think or feel or believe something just because that's what their parents believed? A couple of generations ago, children in Germany were brought up to believe that it was ok to kill Jews and blacks and Gypsies and homosexuals. That sentiment didn't stick around long (mostly). Generations before them were brought up to believe that the best way to cure an illness, any illness, was to cut the sick open and let them "bleed it out." Now we know that that almost never works!
There is new knowledge released into the world every day. To ignore that, and continue a belief system simply because it's the way it's always been done in your family or town or state or country is ludicrous. Not that there's anything wrong with her family. They are really great people, and I fully believe that they raised their children very well. There wasn't anything inherently wrong with whatever it was that she thought either.
It's just that I think people should come to decisions about who they are on their own, as opposed to blindly following tradition.
My friend isn't the only person I've heard say that particular phrase recently. "It's how I was brought up" seems to be the mantra for this area, and probably for most of the world (which, I suspect, is why we're in trouble all the time). Something else I heard recently was (basically) "I don't care what the truth is, I choose to believe this instead." What the hell is that? Why would you choose to continue to believe a fiction, even after you've been presented with the truth? Particularly when it's about something that's easy to prove (as opposed to the cloudy world of religion and politics)? It's very confusing to me, and it bothers me more than it probably should. I'm not entirely sure why I felt compelled to write about it, except that it's been poking me in the brain for weeks now, and perhaps this is a way to get it to stop.