I can only talk about my own motivation for repeatedly bringing this up. If a developer is unaware that they're choosing an inaccessible GUI toolkit for their application, and that application is then required for a particular job, then that developer may end up unwittingly preventing some people from doing that job. And this isn't just hypothetical for me; I know of a blind person who lost his job (luckily only temporarily) because of an inaccessible application. So I think it's entirely reasonable for an application developer to dismiss a GUI toolkit because it's inaccessible. I wish more developers would research this on their own before choosing a toolkit for their applications. But since many don't, I feel obligated to draw attention to it. I'm glad I'm not the only one.
As for your suggestion that applications should implement the platform accessibility APIs themselves: First, it's impractical to expect application-level code to do this directly. The Windows accessibility APIs are notoriously hard to implement correctly. And my understanding is that the Mac, iOS, and Android accessibility APIs are only easy to implement if the application is written in the platform's native programming language. Now, these problems could be solved with an open-source wrapper library -- the SDL or GLFW of accessibility. And I'm planning to implement such a library myself sometime. But my plan is for that library to be used by toolkits, not applications. Because if every application has to directly implement accessibility in parallel with its GUI, then it's a safe bet that even fewer applications will be accessible. We want to get to a place where as many applications as possible can be accessible by default, with minimal effort on the part of the application developers.