It works for people for two reasons:
1) It makes it easy for them to stay in a deficit, so they lose weight. Anything else with a similar deficit would work equally well.
2) They're generally coming from an unhealthy diet to begin with, which is why they feel great on it.
The evidence for vegetable and fruit intake being correlated with better health outcomes is overwhelming. Lower risk of most, if not all cancers, and substantially reduced risk of cardiovascular disease in populations with highest plant food intakes. Fiber intake (only available through plants) lowers blood lipids.
Meat is great, has tons of important micronutrients, and protein intake is very important.
That's no reason to throw the baby out with the bath water.
Eat your fruits and veggies. And your fish, lean meats, all that good stuff.
Consumption of fruits and vegetables, particularly green leafy vegetables and vitamin C-rich fruits and vegetables, appears to have a protective effect against coronary heart disease.
pubmed.ncbi.nlm.nih.gov
Some foods and the vitamins, minerals, and other nutrients found in them may raise or lower cancer risk. Researchers have been studying how the foods, nutrients, and eating patterns listed below are related to cancer.Plant-based foods. These foods contain naturally-occurring substances called...
www.cancer.net
Consumption of dietary soluble fibers has been associated with health benefits such as reduced lipid levels, lower blood pressure, improved blood glucose control, weight loss, improved immune function, and reduced inflammation. Many of these health benefits relate to a reduced risk of developing...
pubmed.ncbi.nlm.nih.gov