Why do people keep saying,"you can't change people" when you can?

Yeah, I hear it all the time: you can’t change people. But do you really believe that? We change all the time. We suddenly start exercising, eat less meat because of information we’ve heard about it’s dangers, go to bed earlier, stop drinking, on and on. There is rehab and self-help books, and a health crisis and all kinds of things that make us change. Yet, we keep hearing, “stop taking this upon yourself. You’ll never change him or her.” It’s wrong, isn’t it?