It's the same at Microsoft and Amazon. In order to work as a contractor, we need to be represented by a managed service, or a preferred vendor. Some vendors offer benefits, others will give you more money for less of their "service". There is some benefit to having it run like this (E&O insurance, quarterly taxes, group health care).

If you are a contractor, make sure you shop around for managed services. There are transparent vendors out there and some willing to show you their cut (although not many).