I’ll try to explain why, using your random variable sampling analogy, which is telling. In the case of say, sampling from a coin toss random variable, there is indeed a more primitive explanation for why the coin comes up heads or tails, it’s just that it would be a much more complicated explanation involving knowledge of huge amounts of detail about the exact force applied to the coin, maybe air currents in the area where coin is being flipped, etc. But in principle that is all explainable.
With quantum measurement, there is no such explanation available, even in principle. We are told only that we will find ourselves in one of the possibilities admitted by the distribution predicted by QM. We have no further explanation for which one it will be. The QM multiverse folks then expect us to be okay with this as the end of the explanation. But notice this is a choice to adopt something as axiomatic. We could have done that at other points in the history of physics but didn’t.