Because there probably hasn't ever been a person who consumed a particular batch of refined nutrients for nearly every meal, and we don't know nearly enough about how the body processes food considering how long we, as a species, have been eating it.

Admittedly, I don't know much about what's in soylent, but I wouldn't be at all surprised if it turns out that eating refined XXX powder in every meal is bad for you, where XXX is an ingredient that he's using to get enough of a particular nutrient.