Because maintaining N different user interfaces is hard.
No matter if the underlying approach or data is the same, the user interface is different and from that point on you have to take them all into account when you make a change.
They're already doing it with their numerous Facebook apps for different platforms.
And it's hard.
And they're still doing a perfectly good job of it.
Presumably at a large expense.
Ridiculously large expense. And not always all that well -- it's just that most people don't use most of the platform simultaneously on different devices / stacks / versions, so they don't notice.