When aiming to produce a product for general consumption, you're either fulfilling an existing demand or you're creating demand. "What do people want?" They want everything. When people want everything, that means they need to be hooked in order for them to even notice your product. These companies are subject to the same ignorant, incompetent and lazy audiences as we are, yet they get people to buy their products anyway.
You don't specify who "these companies" are, but I'm guessing you mean just general companies that are successful doing business in the US.
I don't think you can compare FMT marketing to selling of something like computer games, AI, "life hack" gadgets, vacation rentals, or anything like that, not even (other)
supplements. In trying to sell FMT, you are effectively selling
other people's bodies as a commodity--not directly, but you're selling something derived from those bodies where the body it came from is the very reason it's useful at all. In the US in particular, there is something inherent about human bodies being available as a commodity that squicks people out and gives the product a bad reputation.
Even the companies selling colostrum or milk oligosaccharides from human donors are relying on the fact that they are essentially milk, which many if not most people regularly drink, even if from a cow, and people feed to their babies. I guarantee you that if stool tasted like milk, and it were common to feed it to babies, there would be a thriving stool market because people would demand it.
So it's not so much about making the marketing slick and sexy so that people notice and are "hooked", the issue is what will you market
at all? You can't show the product itself, because what would you show, a beautifully sculpted turd? So then would you show pictures of gorgeous looking donors smiling and telling you how happy they are to provide the product? Well then the bodies are openly the product, and it comes across as if you're advertising an Onlyfans type site if you're not careful. Those companies do good business I'm sure, but do you want that kind of reputation if you're promoting the thing that will hopefully fix the health of the human race? And I'm sure the FDA would take even much less kindly to that than a site like Human Microbes that does its best to look matter-of-fact.
So there's really no mass marketing model for human-body-derived goods that
isn't sleazy. That's part of the problem.