Why can't some people grasp mental illness can't just be "Overcome"?

I was talking to someone last night about the "vagrants" in Vancouver. I pointed out that no one who is capable of not living on the streets would choose to do so, unless they enjoyed eating garbage and sleeping out in the open. I was saying how many of them are mentally ill. Then he got into this thing about how, "We all have challenges in life that we have to overcome." And shit like that. It makes me so upset to see people who have no idea what it means to have a mental illness think that it's something that can just be overcome with a little effort.
As I've said on this board before, I have OCD, and until about a year ago I was not diagnosed or on meds or anything. My life was a living hell, there were days I was unable to function... or had to force myself to get out of bed because I was so deeply rooted in an episode. It goes on and on. I still have very bad flare-ups from time to time. They can be dibilitating and interfear with my work or social life... or even just doing the fucking dishes.
My feeling is that if I didn't have the parents and friends I have, I could have very easily ended up homeless and out of luck.
I tried explaining all of this to this guy. And he just didn't get it. In fact he then used me as an example as to how mental illness can be overcome. I explained to him that I haven't overcome jack shit. That everyday I have to battle it. And that it can be so hard and if you let it take you over, you're fucked. I told him I feel my situation is fairly light compared to what other people may be going through. So, he should cut them some slack.
But, no... there was just no convincing this guy.

It's sad to know there are people out there, who view mentally ill people being disregarded by society, as the fault of the mentally ill person.

I'll take depression over anxiety any day!

but temporary problems.

they are lacking in human empathy, and they are assholes.

I wish mental illness upon them.