HIIT and sprint/shot training is great and all, but most people will quickly shrug it off (or try it for a little while and quit it). Many (I'd argue most) people equate exercise to work, and many people hate work. Instead, most people envy those with fast metabolisms (even in they're "skinny fat" and are, on average, about as healthy as someone pushing obesity) and attempt to copy their lifestyles by becoming skinny with as little work and time investment as possible
without changing eating patterns.
It's the same reason why microwave ownership exploded and is the raison d'etre for the oil change and car wash businesses, amongst many other things. Most people would rather pay a heap of money for other people to do the easy, simple things they can't/don't want/don't have time to do than take the few minutes and do it themselves. If people could pay other people to manage their own weight and make them look good, they would; a
$60B strong weight-loss market proves this. It's not entirely their fault; it's very, very difficult to make time for things with work, kids and wife around unless the body is put first and foremost.
To make matters worse, a lot of people who try to lose weight or "look good for the beach" have horribly unreasonable plans for getting there. ("Looking good for the beach/girls/Facebook" feeds into that.) If many people (I'm tempted to say most) didn't think they could lose 30 pounds in 30 months or get rock-hard solid abs (and nothing else!) in a few weeks, then things like the South Beach diet and P90X would have failed immediately. It doesn't help that lots of parents, who were also very busy and didn't make the time to care for their bodies, didn't make the time to teach their kids proper eating habits and forewent breakfast and simple, reasonably-portioned dinners for McDonald's and frozen TV dinners. (The biggest irony about most people's weight-loss ideologies is that instead of eating the most sugary, fattening crap at breakfast time when their bodies need it the most, they make breakfast the
lightest meal of the day and
eat heavier as the day progresses! Willing to bet it's all because skipping breakfast became a habit.)
Staying in good shape and good weight isn't rocket science and isn't something made exclusive to sports professionals and Olympians. (Funny enough, a lot of these people have HUGE problems maintaining their figure post-career. Michael Jordan and Greg LeMond are kind of fat now, for example.) Eating reasonable portions appropriate to workload and moving around a bit are all that's needed to NOT gain weight. Moving around a LOT and/or eating reasonably is the way to lose weight, and working, stressing and RECOVERING muscles is the way to gain muscle. (Lots of dudes hit the gym and over-stress their bodies with barely enough sleep to let their muscles recover and grow.)
That's it. It's so easy, but is a HUGE mental challenge for lots of people.
Disclaimer: I lost 30 pounds from biking and walking a lot, dropped 3" from my waistline and have been able to keep it off for the last 4 years now. My metabolism isn't naturally great, but I don't think I've ever gained holiday weight.