Spammers, Evildoers, and Opportunists
Look under the hood of any SEO plan and you'll find advice like this: make sure to use keywords in the headline, use proper formatting, provide summaries of the content, include links to relevant information. All of this is a good idea, and none of it is a secret. It's so obvious, anyone who pays for it is a fool.
Occasionally a darkside SEO master may find some loophole in the Google algorithm to exploit, which might actually lead to an increase in traffic. But that ill-gotten traffic gain won't last long. Google changes the way it ranks its index monthly (if not more), so even if some SEO technique worked, and usually they don't, it'll last for a couple weeks, tops.
And when they do reindex, if they determine that you've been acting in bad faith (like hiding links or keywords or other deceptive practices) they'll drop you like a hot rock. So a temporary gain may result in a lifetime ban.
In the end, you're sacrificing your brand integrity in a Faustian bargain for an increase in traffic that won't last the month. And how valuable was that increase, anyway? If you're tricking people into visiting your site, those visits are going to be bad experiences.