The browser you use isn't terribly important though. MS dropped IE for mac 15 years ago.I agree. And I know that Apple is just trying to build their iOS and OS X ecosystem, but a lot of their iOS uses also use Windows and are not about to move to Mac. This would help give them more control over the ecosystem there too.
As Chrome and Firefox work fine on both I can't see any incentive for either company to waste the money as no-one would actively choose either IE or Safari as they are both fairly mediocre even on their native platforms.