makes sense in practice but hard to do for several reasons:
-1. the contractor has contracts with the site that prevents them from hiring you if you have worked with them,
2. The site would rather hire one company who brings in people and vets them vs them having to go out and search for them, set up contracts, pay them etc.
3. liability insurance is bigger with a 3rd party cmpany
4. bascially if you leave, then have to start all over, they pay for the convience of knowing this company will have a person there and working