By doing it that way, you are all the sudden generating tens, if not hundreds of requests per minute to grab notifications for every platform and service, rather than just the one. With a unified approach, the phone can wake up in the background every 5 minutes and ping Google to ask for notifications. If everyone did it individually, your phone would never be able to go to sleep, and would CONSTANTLY be sending out requests to random servers. That also brings up security concerns, since you can get a vague idea of location data from a request, any app that can send notifs can soft track users. They would also open the door for one to be compromised, and send malicious info much easier than it would be to do thru Google. All around, its just a worse solution to the problem with one very small benefit.