I've wondered that too sometimes about the entire IM field - the point you make that if all of these services worked so well, why would anyone sell them when they could make money using them on their own sites.
A lot of it is driven by demand, especially from SEO consultants that have clients. They need to farm out some of the work because of time constraints or lack of specialty skills. Things like bookmarking, social media, graphics, and even offsite content creation; there is a large demand for this, so providers pop up on the supply side.
I'm not saying they are all legit, but it is natural for demand to cause the need for services. Think about how time intensive creating a 2.0 based tier is and all of the associated account & content creation that goes with it - one that looks legitimate.
This really isn't much different from any other business process outsourcing. SEO is the business and there are multiple processes that need done to be successful, depending on the ranking goals.
Google themselves also drives demand. Really, how often do real people naturally share socially or tweet about businesses or products unless it is possibly the hot Christmas toy that year? I don't think I have ever done this with the exception of a game that offered incentives for liking their FB page. So even that was not natural. But these things do have an effect on rankings so we are all forced to pretend and give the impression that people are sitting around all day just sharing, bookmarking, and tweeting about everything they come in contact with.
I agree about what a pain in the ass it is to sift through the marketplace offers sometimes, and how some sound fishy because of the overuse of flashy graphics and !!! and all that. There are times where I'll open one of these and just close them - don't have time to scroll forever in an attempt to find out what the service actually provides.