Because the SEO services offered by most sellers are just part of the solution.
I create content personally (thousands of words a day)
I understand links very well - but use tools and outsourcing for most of them.
People tend to specialise, and have lots of excess time and resource for whatever they specialise in.
For example, my real world business I copyright at £525 ($600) a day - but I often buy tools, links or outsource link creation - though I know exactly what I want from links, and only buy ones that fit my very tight criteria.
If I use a tool or buy a link, it's been well thought out, but even as an SEO of 22 years I buy from other SEO's and sell to businesses as well.
I understand the question, the short answer is "people specialise" and SEO needs quite a few disciplines.
That said IMO some products are crap. Cheap links surrounded by thin/poor/unreadable content on already spam filled websites.
Depends what you are good at.
