Q:

Why don't we completely de-couple frontend JS frameworks and backend APIs? 

Whenever we implement a frontend framework in the likes of Backbone, AngularJS etc.  there's an integration process involved with the backend technologies like NodeJS, Rails, Yii etc. ( like setting up routes, generating views or adding libs etc.)  which varies depending upon what you select at the front-end and what you take at the backend, my question is:
Why don't we completely de-couple the process so that front-end (HTML, CSS, JS etc.)  is completely independent while backend APIs returns us desired data only, just like we do in mobile development. 

A:

This is an architectural choice and you don't need any particular kind of framework to accomplish it, whether on front- or back-end. 
One way to accomplish it (and this is supported by at least some single app javascript frameworks) is to write your front-end with no knowledge or connections to the backend beyond a set of URLs that supply or accept whatever data you require. 
Really, your own answer is correct ;)

