Confirmation bias is my candidate for the top reason. People who succeed (and are most likely to offer "advice") make up idiotic reasons for why they succeeded, mostly discounting the role of luck, and writing self-serving stories and "prescriptions" that are less about helping others and more about self-aggrandization.

More specifically, this comes down to an almost incredible capacity to ignore necessary/sufficient conditions for a particular piece of advice to work.

For me, a piece of advice that does not have an if... then... structure is completely useless, because my basic bullshit detector filter is a "there's no free lunch" rule of thumb.

Almost nobody prefaces advice with the condition, "this will work for you if _________ and will not work if ____________"

The presence of that structure, on the other hand, makes me immediately take the person seriously, because they've taken the trouble to convert a single example (i.e. an existence proof) into a more general truth statement that is carefully circumscribed.

Also a lot of advice that apparently works does not work because of the content of the advice, but the sheer fact of somebody offering an understanding and sympathetic reaction to another person's situation. That alone can be enough sometimes, whether or not the advice is valid. It's sort of a placebo effect. It's what people call "motivational speaking" as opposed to real advice.