A lot of companies reasonably prefer to outsource software work that they don't deem part of their value-add or core competency for the same reason any other company outsources that kind of work. It may or may not be a cost reason (it usually isn't because contractors are not cheap even outsourced ones), it's mostly because you don't want to increase payroll for these tasks or the tasks are of temporary nature.

This is not avoidable, and trying to make it illegal would just kill of American software companies.

As an example: a friend of mine worked for a large consultancy that worked on menu screens for gaming companies. The reason for this is that game companies aren't interested in developing specialized UIs for each game but there are outsourcing agencies that already have entire pipelines set up for this and are basically a sausage factory of game menus for most major game engines.