I think part of it is they’re logical fallacies. For instance, the scientific consensus on climate change is not technically proof of climate change; rather, it’s all the observations, statistics, etc. that are the evidence for climate change. Thus, it is true that claiming an argument is true solely because of scientific consensus is indeed a logical fallacy, as logical fallacies are relating to, well, logic.
For all practical purposes, however, we live in a complex world with lots of uncertainty, and we can generally trust expert consensus if for no other reason than they’re more likely to understand the facts of a certain technical matter better than us, and thus more likely to be able to ascertain the truth. And when discussing complex, technical concepts, I’m generally going to trust expert consensus so long as I am reasonably assured that they are indeed experts and that they have no systemic conflict of interest.