Because there's very little money in research devoted to finding the cause of certain diseases. There's virtually no profit in finding out something that people should avoid and/or eat less, unless you're selling an alternative.
Just look at the research for autism. Scientist openly admit that they don't understand autism well, or why it's becoming more common. But they are terrified to even suggest drugs or vaccines are the cause. Instead they get near unlimited funds trying to find a cure in the form of a pill, because big pharma would make a fortune.
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