Search Engine Optimisation (SEO) used to be quite easy. Essentially you just had to be aware of how Google works on a high level and make sure you had a few key things done right and you could be fairly confortable that you were getting results. It was especially easy for SEO consultants to shine because of much of the competition in some industries was still a bunch of sleeping beauties.
The above was true ten years ago, so in 2006 or so. Since that SEO has become increasingly hard. It's no longer such a mechanical task to get good results. The algorithms that Google and Bing have have grown immensely in terms of sophistication. Leading instead to something known as "content quality ranking", instead of the classic PageRank that was Google's claim to Success.
Content Quality Ranking apparently takes advantage of natural language processing (NLP) and other advanced techniques. This has made the classic functionality of collecting quality links to your website less effective. It's still obviously a great indication of quality, but is also easily exploitable by large commercial publishers who have a vast network of sites, all interconnecting.
SEO still suffers from being seen as a snake oil industry, but the truth is that there's no easy money to be made here any more. At least not in terms of providing real results, nowadays it would be better to take a really long term view on SEO - rather than accepting any offers for quick rewards offered by opportunists.