Maybe they are not competing with them right now but they have aspirations to break into their industry and then compete with them. They figured out it was easier to acquire them than try to build it from the ground up.
Easier to acquire AND easier to divest from if there are any monopoly-related/law-related issues.
ZeniMax competes against XBox Studios or whatever their internal video game studio is called.
Since when was Skyrim a competitor to Microsoft Flight Simulator or Age of Empires?