Um no, and that's part of the problem. In my experience, gym class was a Hunger Games like spectacle for the athletically (but not otherwise) gifted to enjoy their brief decade in the sun at the expense of those who will peak much later in life (but of course, they're more likely to be sedentary in part because of such a negative early impression of physical activity). Evidence? Do I need to mention anything else beyond Shirts vs. Skins Battle Ball? What kind of %$*!ing psychotic thought this sounded like a good idea at the time?
"Why don't we align our medical system with prevention instead of expensive and risky cures?" That's a good question, actually.
No money in it for big pharma, a segment increasingly under attack from the FDA for many of its practices and for which many critical drugs are facing patent expiration. So now they're trying to play good guy by increasingly focusing on orphan diseases ($10K to $500K/year per patient) where if someone blocks a promising therapy, its entire population of victims is doomed. And since these are the long-tail, it also creates an effective barrier to entry for smaller competitors.
But a lot of tenured gym teachers I've met as an adult are the same people who were tormentor jocks in their childhood. So I've never moved further on it.
How do you propose reforming it with people like that blocking the way? Because I think America needs an intervention, Europe isn't far behind, and Asia is doing its level best to catch up right now.
There were a bunch of absolutely awesome ads in the subways of Hong Kong last year that showed a woman's flabby mid-section with a timebomb strapped to it. Contrast this with last week's Facebook kerfuffle between Maria Kang and Curvy Girl which demonstrated (to me) that anything but the most vanilla criticism of this situation in the US is apparently hate speech. Gag me.
Money. Long term care is more profitable than prevention.
In the end I think the answer is "because we, society, allow it.". It is very hard to lobby for things we really do not want.
You could probably answer every question with "People in power use their fortunes to preserve power" and you'd nail it 90% of the time.
Basically, thought experiment: imagine a population where everyone starts with a fixed N dollars. Random-walk each dollar a few iterations and you end up with a bell curve of money distribution. Now start again and instead of random-walking them, apply a probabilistic walk, where having more dollars makes you a proportionately more likely recipient. You end up with a skewed distribution with a few rich controlling most of the resources. Now imagine being a congress-critter in that skewed world. Who are you going to have to cut deals with? And you can make the rich-get-richer rule steeper now, because the dollar-owners are cutting deals to make dollar-taking easier. And now your wealth distribution looks like the real world.
The answer is simply that money itself, the alienable control of resources, is the cause. In a society organized by money, corruption and misrule will result. Any "symptom control" amounts to pumping the distribution flatter - it's a sisyphean task, structurally opposed to the power that money creates, and hence metastable at best.
Maybe you had better gym classes than me, but mine were tedious and boring and turned me (along with most other "nerds" in my class) off from sport until I reached high school and discovered weight lifting. I always liked Kelly Starrett's motto of "all human beings should be able to perform basic maintenance on themselves" and I wish that point was more emphasized during physical education.
I don't know of many schools in my state that still have the budget to provide a home economics class/pay a teacher to teach it. The most my school went into nutrition was in the 1 semester of health class that everyone took.
Their goal is to reduce the cost of treatment by encouraging prevention. I'm not sure how many people make use of it.
"Why don't we teach effective nutrition and cooking in school?" We do. It's called home economics.
"Why don't we teach effective exercise, sports, and fitness in schools?" We do. It's called gym.
"If we can ban alcohol and tobacco sales to children, why not things like added sugar, hydrogenated oils, corn syrup, etc?" Go tell a 6 year old they can't have cake on their birthday.
"If we can ban alcohol and tobacco advertising to children, why not products with added sugar, hydrogenated oils, corn syrup, etc?" Lobbyists
"Why don't we limit advertising on fast and junk "food"?" Lobbyists
"Why don't we provide healthy lunches to school children instead of using them to subsidize agricultural conglomerates for low quality food?" We kinda do: http://www.fns.usda.gov/nslp/national-school-lunch-program
"Why don't we stop subsidizing fossil fuels so much so people would walk and ride bikes more?" How many times do I have to say this: LOBBYISTS.
"Why don't we align our medical system with prevention instead of expensive and risky cures?" That's a good question, actually.
EDIT: I like falcolas' answer: Money. Long term care is more profitable than prevention.