It really is simple if you want to understand why guaranteed results SEO is always a scam.
If someone really had the super SEO skills or the inside scoop on the secret algorithms of search engines and they could guarantee ranking, then why would they do it for $50, $500, $5,000 or even $50,000? The top spot for a competitive keyword (top 500 revenue generating) can be worth six figures easy, so why not do it for themselves and retire? Why do it for you on the cheap?
Besides this obvious reasoning you might wonder why anyone would be so desperate for business that they would take a chance of working for free? Most often they are either using paid search to place you on the first page and ripping you off, or they are ranking you for non-competitive keywords and ripping you off.