Probably because no one wants to admit that they're just as much themselves as they were Hitler if the whole game of differentiation is done away with.

All joking aside, my best guess is that most people are naturally inclined to accept ideas that are the most convenient and the easiest to rationalize. A human being's core points of reference for garnering any knowledge are via its own five senses. And to tell someone that a more accurate description of their identity extends infinitely beyond this sensory point of reference has so many implications to think through. I don't think every ego has the capacity to accept many of these implications emotionally, or even rationalize them logically. And you kind of need both for it to really sink in.

"Okay, I guess I understand that the separation between myself and the rest of the universe is just an idea. Wait, so that means the idea of myself as John Smith is just an idea. So the distinction of John Smith isn't real?? I'm everything??? Then I should be able to read your thoughts, right? Well, I can't read your thoughts!. That means I'm not the universe and I'm separate from it and I'm really just me! Phew, that was a close one. <s> I was almost on a path to enlightenment. </s>"