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.
The problem with that is that fortification is already rampant in premade foods, and so most first-worlders are already eating these refined ingredients in every meal. Moreover, many people take one-a-day multivitamins which in a single dose, contain the same amount (and sometimes more) of the same refined ingredients in a day's worth of soylent.
Extensive studies have been done on the safety and efficacy of multivitamins, and while their utility for healthy people with balanced diets is a matter of debate, their safety (when taken at the recommended dose) is well established.
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.