There is a great conversation to be had about whether this is an emergent property of market forces, or whether this is designed intentionally, and how much so. 

Centralization is a very straight line way to solve a ton of problems and provide seamless integration. And, it is quite possibly the only way at all to be able to control the experience in the sense of making the experience of using something the same every time. 

Most integrations in the mobile market make absolute sense to me. And usually I have a difficult time seeing a way such things could be done in a decentralized fashion without some pretty major tradeoffs.