Also, generally, it's a way to express solidarity with the reality that a lot of time there's no "hoping for good news", "buck up" is a horrible thing to say, and just... well, fuck cancer.
There is an unmeasurably large amount of suffering due to cancer. Not just from the people that have directly experienced it. Watching helplessly as it consumes the bodies and minds of your loved ones is devastating. Its likely that someone expressing that sentiment has dealt with that fallout. My heart goes out to them.
I don't know if it's thought of in popular culture as "coming from" a specific source, but I know people have been saying it for a lot longer than the internet. It's been independently coined a million times at least. It's really the only thing you can think to say sometimes to sum up the experience for everybody involved.
I think until you have someone close experience it you can't truly appreciate the sentiment. From my experience, having someone go through it is an awful, helpless time. I've heard having parents go through alzheimers is similar.
Also, generally, it's a way to express solidarity with the reality that a lot of time there's no "hoping for good news", "buck up" is a horrible thing to say, and just... well, fuck cancer.