It really all boils down to risk management. If you're a self-identified say, "Ruby freelancer", the value proposition is "I am an independent Ruby developer for hire". For staff augmentation gigs or applying for a FT position, this is all well and good, because whoever's hiring is looking for warm bodies with specific technical capabilities. However, many companies are looking to solve specific business problems and…
OTOH a customer with a bigger budget who is looking for a more holistic solution may also want infrastructure provisioning and management, SEO and color scheme designed specifically to appeal to their target demographic. It's unlikely that any single person can do all of these things, at least to a level of quality that would justify a significant fee. So either you need a reliable bunch of subcontractors who you can retain for X days/month or you need to partner with somebody with complimentary skillsets.