I think the most plausible reason is the same reason that Microsoft and IBM don't sue each other over patents.
Apple and MS have so many core patents that cover Windows, OS X, Windows Mobile and iOS, not to mention media players, office software etc. that any lawsuit will most probably end up in huge legal fees followed by a cross licensing deal after all the counter claims are decided on.
Apple and MS have so many core patents that cover Windows, OS X, Windows Mobile and iOS, not to mention media players, office software etc. that any lawsuit will most probably end up in huge legal fees followed by a cross licensing deal after all the counter claims are decided on.