In a way, it's similar in nature to how people will, with best intentions, tell you to smile more if you have depression.

I think the issue is many people don't understand the problem and they can't see it. In fact, someone with a common cold can expect to get more sympathy and better advice than someone with a chronic illness. The second issue is that many people struggle to think of something to say, its quite uncomfortable and the automatic choice is to give advice.