It's more about the nature of the problem, not that you can't do it in shaders. After all, I think you can do pretty much anything in shaders if you try hard enough.

Even if you already have a GPU renderer for glyphs and any other vector data, you still want to know where to actually position the glyphs. And since this is highly dependent on the text itself and your application state (that lies on the CPU), it would actually be pretty difficult to do it directly on the GPU. The shader that you would want should emit positions, but the code to do that won't be easily ported to the GPU. Working with text is not really what shaders are meant for.