" Why do American voters expect their "politicians"(be it democrat/republican) to stop offshoring, when these guys seem to have absolutely have no control over individual companies doing that"

They do have quite a bit of control. The companies are doing it primarily because they lobbied politicians to pass laws and treaties to make it easier for them in the first place. Such things can be reversed although the consequences will be huge. Shouldn't be done suddenly which is what I don't like about what he'll probably do.

Far as voter expectations, those against globalization see the government there to meet their needs. This includes protecting their jobs and wages. The offshoring has been negative for those here while positive for them elsewhere (eg India). Incentivizing companies to hire Americans and pay them well increases quality of life here plus gives us stronger, internal economy.