Most people are not contractors with an established company/legal status as contractors, so you want to hire them. To "hire" someone formally for a week, though, entails a lot of paperwork and has, depending on the country, legal consequences (e.g. voting rights in worker councils, minimum employment durations, right to sue for wrongful termination)...
The optimal way would be if lawmakers could create some sort of "employed contractor", however I can think of a dozen ways for employers to screw over poor people with it...