Because they get a working IOS / Android hardware compatible port for free. If it runs on the Switch hardware, there will be almost no effort to release it on mobile devices. And also porting existing mobile games to switch is easy too.
Porting Skyrim to Switch alone is not worth the cost and effort at all because sales expectations are very low.
But porting Skyrim to Switch and also releasing it on IOS / Android Appstores will create a lot of revenue and is worth the effort.







