ChristianKI is not telling you to trust a domain expert, but rather to read / listen to the domain expert long enough to understand what they are saying (rather than instantly assuming they are wrong because they say something that seems to conflict with your preconceived notions).

I think if you were to read most machine learning books, you would get quite a lot of "why". See this manuscript for instance. I don't really see why you think that Bayesians have a monopoly on being able to explain things.