A guy on reddit who said he was a medical student (and sounded like he really was) said that the nutrients may be fine, but the digestive system needs solid food. Patients stuck on liquid diets tend not to do well long-term, and they try to get them on solid food as fast as they can. He had a technical explanation I don't remember well enough to repeat.

I asked how the Soylent guy would do if he converted the stuff into some kind of solid food bar, and the med student said "Probably much better!"