* Most big companies simply won't need that very specialized know-how for longer than the project duration. So it's better to just overpay a bit for the duration of the project and then cut off all ties to the worker.
* Finding good people to work on important projects is very difficult and entails lots of money/time effort. (Either to find people or to train them.) A consultancy or engineering services company guarantees that the people they send are qualified. They also guarantee that they will provide replacements for people who quite or get ill. They might ever provide a project manager, QA people etc. that make it all much easier for the customer.
* Most big companies simply won't need that very specialized know-how for longer than the project duration. So it's better to just overpay a bit for the duration of the project and then cut off all ties to the worker.
* Finding good people to work on important projects is very difficult and entails lots of money/time effort. (Either to find people or to train them.) A consultancy or engineering services company guarantees that the people they send are qualified. They also guarantee that they will provide replacements for people who quite or get ill. They might ever provide a project manager, QA people etc. that make it all much easier for the customer.