Much of it driven by cult cargo SEO, throwing everything and the kitchen sink into the page in completely unproven hope that it'll somehow game the rankings
It is cargo cult but it's cargo cult because it is the way to "success". Company A have great page ranking, and blog about how they think they got there. Company B also have great page ranking, but think they did something different to Company A, so they blog about it, too. Everyone else reads both blogs and intersects what both companies did, and implement those changes. Iterate for every difference you encounter and voila.. you now have your rubber stamp SEO method.
Even (or rather, especially) if every SEO advice is correct, it still means that Google effectively has a lot of control over the shape of the modern web, alone through indirect pressure via SEO.
