Because it creates a conflict of interest, where their income depends on the continued significance and validity of their research results.
And when a researcher this fanatically believes in their own research, were they properly free of biases going into the research? Is writing the book the consequence of amazing research results, or did they plan to write the book from the start (maybe seeing it as the only way to make investment into the research worthwhile)?
The conflict of interest is always there since almost any scientist wants to discover something and be right. That doesn't mean everyone does bad science, but it's not possible to be unbiased.
I'd say the motivation to name a phenomenon and make a mark in the science history might be much higher and more widespread then to sell a book.
Science cannot depend on scientists being unbiased about their work. It has to depend on verification, reproduction of experiments, meta analysis and so on.
Scientists are likely to be assessed on the basis of their general credibility, not the standing of a single paper.
Maintaining general credibility typically requires not overstating your claims, especially when dealing with sparse data.
Publishing a single paper and then making money on promoting lifestyle changes based on your discovery is not science, and would be (I would say correctly) side-eyed by many scientists. The author’s credibility is now inexorably bound up with a single statement that has been reduced to marketing.
There's an incentive for the work to be right, explainable and having transposable advice.
A common thread I see in discussions about age and about weight gain is about how a lot of it is determined by genetics. If you are trying to sell a diet book, are you going to dig into the genetics part a lot? Probably not![0]
Flavors of this exist in all domains, of course, and it's not that the causal relation is "writing a book on the effect makes the research bad". But when you show up with a problem and the solution in one package, there is a question about whether this is research or whether this is a sales pitch (likely something in between).
[0]: not taking a position on the actual veracity of the genetics back-and-forth.
If one does a good deed, but does so with the intention of telling others about the good deed to gain social status, is the deed still good despite the ulterior motive?
Isn't the point of starting a business to make money off a product that is useful to people and makes their lives better, hence people willing to trade their money for that product? If healthy living can be a product, what's wrong with monetizing it?
I think the problem is that there is an obvious incentive for the author to achieve research findings that support their enterprise, and ignore the facts that are contrary. It's an opening for corruption/conflict-of-interest.
That's not to say that it's not possible that they are both correct in their findings and able to make money off it, but it does reduce their credibility.
Who's advice do you take more seriously, the guy with a horse in the race, or the guy without?
The problem is trying to monetize something new and unproven by claiming some kind of significant virtue.
Charging money to provide value under conditions where it's a proven and known means to add value is not controversial. But merely virtue signaling on something you can't prove is a common tactic of con artists.
The Bible seems to think that not a good thing.
"But when you do a charitable deed, do not let your left hand know what your right hand is doing"- Mathew 6:3.
"The problem with the middlebrow dismissal is that it's a magnet for upvotes."
Currently the top comments are what pg was worried about (I'd say). This is an old concept. The top comments are not pulling out any big guns to say why it's wrong but are getting upvoted.
And so... https://www.amazon.com/dp/1426209487
They usually also have a company with the same name that sells speaking engagements or consulting... https://www.bluezones.com/
And of course, you need a modestly astroturfed Wikipedia article... https://en.wikipedia.org/wiki/Blue_zone
And don't forget a TED Talk! https://www.ted.com/talks/dan_buettner_how_to_live_to_be_100