Why is a PhD thesis typically 150 pages? I just don't understand why the overall findings &amp; implications of a PhD cannot be communicated in the style of a research paper?

Succinctness is a virtue in a field characterised by time pressure.

The research output is equal to or less than that of a 3 year postdoc - which is always communicated as a research paper.

Just seems like added embellishment for the sake of embellishment.

What am I missing?
I don't know where your 150 page idea comes from since I have never heard this as a guideline. However, in my field, "staple theses" are common, composed of roughly 3 papers stapled together (either already published or publishable drafts), with an added introduction and conclusion that tie the works together and may get a bit deeper into background than is acceptable in a published manuscript.

Therefore, they must be at least the length of three research papers, and aren't typically all that much longer than that. However, theses are also typically formatted in a longer form, more similar to the original word-processed manuscript, whereas journal articles are composed in a dense, newspaper-like format. Figures may have their own pages. In sum, a manuscript that is 10-15 published pages can easily be 30-50 pages in this format.

No embellishment necessary to get to 150 pages.

I'd also add that a post doc in my field who only produced 1 paper in 3 years would probably have their next job in industry; a graduate student with only one paper will have been quite disappointing (it may happen that only one paper is publishable due to being scooped or unexpected failures, but even those outcomes can be part of a thesis). Not necessarily, it's possible that paper could be highly impactful, and might be reasonable in some subfields, but generally that would not be a suitable output for an academic career.



Edit: decided to pull up my own thesis from the archives. 230 pages. Of those, 53 pages are "thesis-only" pages; the other 177 pages are verbatim copies from 2 published papers and a third draft manuscript which was later revised and published, all reformatted to fit the thesis formatting requirements. Of the remaining 53 pages of "fluff," 6 pages are title/contents/acknowledgements/abstract, and about 19 are references for the introduction/conclusion chapters, so about 28 extra pages of generously spaced writing, to be treated as "embellishment" if you wish, or alternatively, to provide enough background and context for the work for someone familiar with the field but not previously familiar with my research area, such as members of my thesis committee besides my advisor.

In summary, even if my thesis had nothing but 3 published/publishable manuscripts, it would already be >150 pages.