Earlier quoted context omitted.
I've always disliked it too. It seems very presumptuous. The people repeating it typically don't know what the person in question is dealing with.
actually that statement is itself rather presumptuous. How would you know that those of us who repeat it don't know? I can state with authority that may of us do.
Because everybody is different and even if you have been through it yourself, your experience may be nothing like someone else's, and just because it was true for you, doesn't mean it is true for them. Yes, some people get past it, either on their own, with therapy or with medication. Some people are just numbed by it, other people learn to cope even if it never actually gets better. Other people still struggle every day. You cannot know what is going on in someone else's head or the struggles they face.
Its true that many people get better, certainly with treatment, but a large number of people do not (someone else here said that treatment does not work for approx. 1/3 of people).