Speaking as someone who has ported games to other platforms before, it's not as easy as you say. There's almost always platform-specific issues to deal with. It

Speaking as someone who has ported games to other platforms before, it's not as easy as you say. There's almost always platform-specific issues to deal with. It's rarely as easy as "Export -> to Xbox" (although Unity tries real hard to do this, it took us several months to convert one of our games from PC to iOS). Differences in input, resolution, screen sizes, and RAM often become hurdles that take real time and manpower to overcome, and can be insurmountable, depending on how the game is structured. The companies might have legitimate concerns over whether their time is better spent on porting to new platforms, or spent working on the next title. That's why you see so many third parties handle the ports, so the core team can move on to new projects.