I worked for two good outsourcing shops and both experiences were great.
This is not true.
The freelancer simply needs to position himself as someone who can build and manage a team that can do that. I've served as a technical lead on projects managing other freelancers. Large projects.
While that might be the case for some clients or projects mostly the problem is a sales and legal one.
Single freelancers can't afford to deal with ridiculously complex enterprise sales processes.
Large clients on the other hand for the most part don't want to be directly involved with contractors due to administrative (managing accounts payable for a huge number of small suppliers instead of just a handful of large suppliers) and legal reasons (managing and assessing individual supplier contracts; depending on the country and how you market your services as a freelancer there can also be issues around fake self-employment, which this way large clients can conveniently outsource to a third party).