"So, if they are happily opening offices overseas to save on tax anyway, why not employ workers in those companies and get them to collaborate with the parent company?"
Latency is a technical requirement. High salary tech workers need low latency connections to the services in the US. Additionally, most of the technical expertise is still in the US. You bring them in slowly, have the US tech workers with the knowledge and the skills train them. Then you use your high labor costs as an excuse to lay off the US tech workers you just had train the imports and get them fit for interacting with US management.