Even if the project does not begin as a scam it can easily turn into one, because even if they were to back their stable coin with fiat the ratio they promised their investors which will be maintained can always be altered over the life of the project, this means that people are taking a huge risk by investing in those projects, and what is worse is that those coins offer a sense of security to investors and they may not realize that instead they are taking the biggest risk on their short careers.
But you don't take into account how many such projects have actually lost their investors money. Which major stablecoins have collapsed recently and lost their tie-up completely? UST? Which others? There haven't been any more such high-profile failures. Yes, stablecoins can instill a false sense of stability and bankrupt their customer, but how many such cases have there been among the big stablecoins? Few. That's why stablecoins are still a less risky investment because they go bankrupt very rarely.