Because that is actually the classical pattern of applications using the internet ;-) The whole point of internet protocols like HTTP, SMTP and the likes is about not having control over the server and being able to let programs talk to each other anyway.

So if you want to build a classic client app it would be independent from the server implementation. So maybe it would even be 'your' that you would use. For example you could built an app that could talk to a Dropbox server. But with the current SOP setup you would require Dropbox to configure their CORS accordingly.