The primary challenge with feeding soldiers in the field is not efficiently delivering nutritionally complete food to them, it is getting them to eat it day after day after day. Soldiers get bored eating the same things over and over again and eventually this drives them to stop eating, which negatively affects their performance.

It was technically feasible to replace rations with meal replacement powders decades ago, but the reason that MREs continue to be a variety of real food items is because they are more likely to be eaten that way. Even with all the variety, most US and Canadian rations include hot sauce to add optional variety to the same set of packaged meals for those who happen to eat a lot of them. This is also why MREs include heater bags - because hot food is more likely to be consumed than cold. Incidentally, this is also why personnel tend to love other nations' ration packs and disparage their own - not because they're better, but because they're not sick of them.

Food Officers take this stuff pretty seriously, and put a lot of effort into delivering nutritionally complete food to soldiers that they will actually enjoy eating. Meal replacement powders don't cut it in this environment.

As for Soylent, the primary use case I have seen is convenience. In this role, it's better than many other convenience foods, so I can see why some people go for it.

Example: In my case, I don't particularly feel like cooking breakfast if I get up early to play golf or hike, and doing either while hungry is unpleasant. Soylent solves that problem, and I prefer it to Clif bars, which is what I would have used before Soylent.

Absolutely nothing. But it's not uncommon, if you skip a meal, to feel a bit low energy and/or to snack instead. When I'm out doing things, I'll often skip lunch but I may have a granola bar, fruit, some nuts, or something along those lines.