Mostly because it's being used in the same way as "think of the starving children in Africa". Of course there are people that are much, much worse off than us but if any comparison should always be towards the lowest possible bar then you'll lose every time. Particularly if you throw in history on how growing up today is much better than most children through history, probably including your own parents and grandparents. After all, most people - certainly kids and other young people you identify with - do have their health.
Also it's sometimes used as a poor man's equalizer, it doesn't matter that you're Steve Jobs you can still die a long drawn out death of cancer. In that yes your health is important and your health can't really be bought for money, but just because there's a variable you can't control doesn't mean poor and (good|bad) health beats rich and (good|bad) health. It's a just a way to mentally put a few people in the (rich, bad health) below you (poor, good health) in the feelgood hierarchy.
If this is intended to make you feel good about making poor choices, then carry on.
However, I'll tell you now that most people under 30 are typically living in a dream world. "Poor health" is a concept to most such "youngsters." When I was that age I'd been ill and I'd been injured, and I thought I understood. But, now, with the mild aches and pains of age creeping up on me slowly, I realize how big that gun is that I'm looking down the barrel of. Poor health isn't about being hit by a taxi-cab at 9 and getting my skull fractured. It's not about getting walking pneumonia at 19 and having to walk a mile to the hospital for treatment. It's about being in pain (or even just discomfort) and knowing that you're going to feel that way for a very, very long time, if not the rest of your life.
Not that I'm that bad off. I have a few minor aches and pains that are the sign that my body has stopped being forgiving about trivial injury. But it does put some things in perspective.