Apple has a history of adding apps like this all the way back to MacWrite, MacPaint. Their strategy has been consistent since the beginning of the Mac; they provide end to end experiences that work out of the box. In the case of collaboration the use case has become so essential to their experience that they needed to build it themselves instead of relying on third parties. They do the work of integration instead of asking the user to do it. The downside is that the cross-Apple experiences are sometimes not great but that's not the problem they are trying to solve.