Well, the problem is that every would need to have their own server with notifications waiting to be pulled (imagine your phone goes offline) and they need to be beefy enough to answer potentially thousands of requests per second. Almost impossible for small devs.

There's also additional battery need, as it's many calls and payloads, and if a server is slow it can affect all the other notifications. Plus more area of attack.

Not impossible, but I don't think it's the direction things will go.