There's no reason in principle why an OpenGL-based GUI must be inaccessible. Qt heavily uses OpenGL, at least on some platforms, but is still more or less accessible.
It's more accurate to say that the vast majority of GUI toolkits that have ever been written are inaccessible, and most OpenGL-based toolkits fall into this category. Accessibility is tedious to implement at the toolkit level, especially in a cross-platform toolkit, so generally only big toolkits with corporate backing get it.
Also, on the web platform, if you abandon semantic HTML and use canvas or WebGL, then you're forfeiting the accessibility that your UI would normally have by default. As far as I know, the only way to make a canvas or WebGL-based UI accessible is to construct a parallel HTML DOM tree (and presumably use CSS to hide it behind the canvas).
It's more accurate to say that the vast majority of GUI toolkits that have ever been written are inaccessible, and most OpenGL-based toolkits fall into this category. Accessibility is tedious to implement at the toolkit level, especially in a cross-platform toolkit, so generally only big toolkits with corporate backing get it.
Also, on the web platform, if you abandon semantic HTML and use canvas or WebGL, then you're forfeiting the accessibility that your UI would normally have by default. As far as I know, the only way to make a canvas or WebGL-based UI accessible is to construct a parallel HTML DOM tree (and presumably use CSS to hide it behind the canvas).
Thanks for being curious about this.