I mean, much more often than not, and for the majority of the time, they are.

You don't see this statement as dogmatic? How do you feel confident in this other than just a feeling?

The majority of the time the articles would require actual expertise to make that evaluation with confidence. An individual can take a few minutes to verify the sources, but for so many topics it's not realistic to rule out omissions of sources that should be well-known, or even rule out that a source given provides an important broader context somewhere nearby that should be mentioned in the article but isn't. Can you be sure that the author is trustworthy on this subject? It's not enough to just check a single page mentioned in a book while ignoring the rest of the book and any context surrounding the author.

An expert on a very specialized topic could weigh with accuracy in on whether the wikipedia articles on their subject is well-researched and sourced, but that still won't mean they can extrapolate their conclusion to other articles.