Because if something goes wrong, the things you are breaking will be people's well-being. There were two instances where Rob noticed that he was feeling ill and had to correct a nutritional deficiency on the fly. It's less likely that this will happen during large-scale production, but if it does, the people consuming exclusively Soylent will not have all the knowledge Rob did wrt the formula or the symptoms of nutritional deficiency.
I think Soylent is a good idea, and ordered a week's supply, but I'm going to try it slowly; I think the chance that they screw up the production is large enough to merit caution.
I agree that nutritional deficiencies are a problem to watch out for but disagree that consuming Soylent would vastly increase my risk of nutritional deficiencies relative to my current diet. I don't think you're taking into account how bad many people's current diets are / could be. The question is not whether eating Soylent is dangerous but whether it's substantially more dangerous than whatever people are already eating.
Rob Rhinehart's food replacement Soylent now has a crowdfunding campaign.
If you're interested in one or more of these benefits, send in some money! There is also a new blog post.