In fact, all these fears of players came to gambling from earlier times when honesty was not controlled by mathematics and evidence, but by regulatory authorities (or no one at all) and, accordingly, there was a huge field for manipulation.
Now sometimes you can also find gaming sites created by scammers. These sites do not exist for long, but they manage to deceive a certain number of people. Therefore, people who have passed through the hands of such scammers will be cautious about other online casinos.
As you correctly noted, these are one-day projects and they are unlikely to deceive anyone for serious sums - software development, website launch, advertising (even illegal) are expensive compared to the earnings they will have time to get. Therefore, for purely economic reasons, such projects either live very little or do not appear at all.