Love for the material isn't the only possible motivation; people need motivation and discipline to stick with it, but the motivation doesn't have to be love of the material and there's no reason that that motivation or discipline has to be integrated. Some people will bring their own (of both), people that have the first can get the other by any number of means. Sure, there are programs that package everything (except probably basic motivation) together, and sure, they are good for some people, but they aren't necessarily ideal for everyone, and certainly aren't always the most cost effective for people that have their own motivation and discipline.
Like always getting into shape is mostly about not eating rather than exercising. With our highly processed food one could do nothing but run marathons day in day out and still be morbidly obese.
If research that goes against the interests of a huge multinational oligopoly has an r value less than 0.1 it's probably true.
Saying that you should just exercise more is literally an opinion taken from a Coca Cola commercial: https://www.theatlantic.com/health/archive/2013/01/cokes-unc...
In the end it takes the person pushing themselves. And unless the person has a real love for the material, people just won't. And it's kind of lousy that this is treated as a horrible personal failing when it's pretty much how people operate. Even a poor teacher at the very least adds accountability.