Why do some journals prefer MS Word? Some journals (specifically on biology or chemistry) accept only MS Word files as manuscripts, while many journals on mathematics or physics prefer LaTeX.
Even Science prefers MS Word.

Why do they insist on proprietary MS Word, instead of accepting PDF + plain text?
I don't think they typeset papers on MS Word.
I cannot answer for all journals but can provide insights into a few. One issue concerns the authors rather than the journals themselves. If a community has little need for equations then the need for LaTeX is also perceived as small. Hence a journal would not implement something that they think has no use.

In my own field of Earth Science, the usage of LaTeX varies between sub-disciplines. In Glaciology, being a quantitative subfield, a majority use LaTeX while in other more descriptive subfields no-one uses LaTeX. It is easy to see that this is also reflected in what formats the journals accept. Since the editors of the journals usually come from the sub-disciplines their journals cover, the editors are used to using the same authoring tools as others which further cements the existing structure.

On top of that the individual publisher may add capacity to journals. Elsevier, for example, has a LaTeX-class that can be used for their journals, while Wiley, for example, do not. This can help journals take the step to also include LaTeX contributions even though editors may not be users themselves.