While I think in theory it's possible for them to work--and they might indeed work for specific people with specific needs--a percentage of people using them are probably of a similar type to others who have gravitated towards food fads through the past century.

Like, if you hit up the Wikpedia or some history site and look at food/diet ads from 100 years ago, those products look pretty ridiculous to modern eyes. But they're marketing the same thing, right? Health? Convenience? They're targeting people who are desperate for solutions to their problems, using marketing language common to that era.

And I think a large percentage of these meal replacement products are doing the same thing to modern people, that all the "health food" stuff from decades prior did to our grandparents and great-grandparents. People are, after all, people, and it's easy to fall for marketing regardless of what era you live in.