The collective action problem of job search advice
It's understandable that I want such advice -- I want to get a job. And it makes sense why my advisors and close colleagues would offer such advice specifically to me, since they have both personal and person-regarding-altruistic motives for wanting me in particular to get a job. To offer such advice to the world at large, then, would seem a natural extension -- you want everyone, not just yourself and your personal friends, to get a job.
This kind of extension makes sense for non-competitive goods. It's sensible to offer teaching advice to the world at large, because every professor could become a better teacher. The same thing goes for advice on doing good research -- it's sensible and desirable for everyone in academia to do better research.
But job-hunting is different in the crucial respect of being a zero-sum game. There are X jobs available each cycle, and each job will be filled with exactly one candidate. Thus, offering advice to all job-seekers doesn't increase the number of people with jobs. If we assume, for the sake of simplicity, that all job-seekers read the advice, all it does is intensify the level of competition.
The question, then, is whether intensifying the level of competition is a good thing. There are some situations in which more intense competition is intrinsically good -- after all, there's a reason that most players and fans prefer major-league baseball over little league, even though both leagues have one winner and one loser per game. This seems unlikely to be the case in the job market, since I've never heard of either a job-seeker or a selection committee member who loves the process itself.
Another possibility is that the competition creates better candidates than would have existed otherwise. The model here is Adam Smith's idea of the market, in which competition drives businesses to improve their products. Tougher competition in the academic job market may, for example, drive potential candidates to work more on improving their teaching skills, which will make them better at the job if they get it. I doubt that this is a primary reason for most academics to improve their teaching and research, but it's not insignificant (it was, for example, one element of my decision to take my current adjunct job).
It's also possible that more intense competition could also improve the committee's ability to pick the best candidate, by making the competition more informative. Some of the advice seems well-suited to this -- for example, advice on writing a better CV will lead to CVs that more clearly explain candidates' qualifications, thus enabling the committee to more effectively pick the best candidate. Something similar could be said for advice that helps candidates stay calm during interviews, thus enabling them to show their full qualifications.
However, much of the advice -- and, it seems, the most actively sought advice -- does not fit any of the above categories of advice that's beneficial at the social level. I'm talking here of the "tips and tricks" genre of advice. This kind of advice aims simply to help the candidate advance their own personal prospects, even to game the system in a sense. For example, one article (I've lost the links to all the things I've read) talked about choosing the right outfit to send a certain message to the committee about yourself. This particular piece of advice is interesting because in a sense it goes beyond merely being futile when offered to all candidates (as opposed to just to one candidate whose victory you favor) into being self-defeating. The committee is (consciously or unconsciously) judging candidates' clothing because they believe it reveals something important and informative about them. But after the advice is given, all wearing the right outfit is actually revealing is whether you've read the advice article.
Then again, perhaps I'm looking at it wrong by assuming that the writers of such articles are primarily motivated by making some social-level improvement in the job search process. They may be writing mostly to drive traffic to their website (which will increase as the job competition becomes more intense and thus candidates need to keep up on the best advice) and/or to increase their prestige as someone who gives out advice.