What's wrong with those microwavable stuff (like noodles, and others)? I mean it's not healthy, but if it's once in a while, it's cheaper and imo tastes better!
I'm guessing you and many other commenters haven't hit the age where heart disease, high blood pressure, and pre-diabetes start raising their ugly heads. All of those conditions are depressingly common in the Western world and require medication with various side effects to treat. Each one of those microwaveable meals, though tasty and convenient, takes you one step closer to a rather unpleasant conversation with your doctor.
Given this, it should hardly be surprising that there's a market for a convenient meal replacement that is, if not actually healthy, then at least health-neutral.
("Just prepare your own healthy means and exercise more!" you say? Easier said than done. Just because you are able to do it doesn't mean everybody is in a situation where they can do the same.)
And that's precisely why people turn to Soylent and its competitors, because they are attempting to be healthy, balanced sources of nutrition. A bowl of instant ramen might be tasty but it's not going to get you much in the way of protein or vitamins.