tl;dr They don't know.

I wish scientists would be more open about how little they understand digestion and nutrition, particularly on an individual level. Advice gets presented as an if-then, when it's not.

If you have a complaint against "scientists" as hsme homogenous group, I think I'm going to have to ask you to explain how these particular scientists did not do that, and why you would think this is a problem of scientists (a label for a largelt disparate group not connected through any specific communication or hierarchy and mostly in output) in general?