Aren't games pretty damn portable these days? It seems like most non-firsty party titles are all multi-platform now. I don't know anything about game development but they must be developing with portability in mind.
As suby mentioned, off-the-shelf I think these engines support multiple platforms, but it is surprising (to me) how many big, non-1st-party games don't make it across all the platforms in a timely fashion. That signals to me that it isn't as easy as we might think.
