Tooling and IDEs
Of course, in some respects, this is a problem with tooling rather than with the framework. I suppose that if I could point Eclipse or Netbeans at a REST service and have it generate the code for say, an Angular 2 service module that accesses the service, I might feel differently. Or if I could generate a first-cut form to do CRUD with that service, I'd feel better about these frameworks. I also get the feeling that developers WANT to code it, and hate handing over part of the job to someone else, especially with something like ADF, where you can't easily see the source code.
Responsiveness and Speed
Another advantage claimed for these environments is that most of the processing takes place on the user's computer, and interacts directly with the browser. Since the application doesn't have to interact with the server as much, it runs faster. And on the server side, it isn't as complicated and doesn't require as much server resources. All the server is doing is downloading files to the client, and running REST services. What's more, the browser usually caches files that it has already downloaded, so if the file hasn't changed, it will use its local copy of the file, and won't go to the server for a fresh copy. Please note that this can be a disadvantage for mobile devices with slow connections and limited storage. There is a lot more code to be downloaded and stored.
Testing and Security