The first bonuses given to newcomers are a source of motivation to play and remain committed to the site. Why some people who doesn't have much capital to bet benefits from these bonuses. Some who has money for betting doesn't care of the bonuses. What they care is if the site has the options and they are reputable enough that they will be able to pay out winnings when they are big.
Nothing feels good then knowing that you are dealing with a reliable betting company.
While that is what people should care about, in my experience the majority of gamblers care about the bonuses they are going to receive to the point they ignore a great deal of the red flags those casinos show, and it is why we have so many people being scammed by casinos which have had a bad reputation for a very long time, so while I agree with you and the reputation of the casino should be the priority when selecting a casino in which to play, the reality is that most gamblers do not care about that.