> Explain to me exactly why, other than 'I guess someone already implemented some kind of basic version of it' that you would have to have custom CPU code rendering glyphs instead of a shader rendering SDF's like literally everyone does with shaders already?

Shaping is different compared to rendering glyphs themselves. SDF renderers (and other GPU text renderers like Slug) still do shaping on the CPU, not in shaders. Maybe some experiments have been done in this area, but I doubt anyone shapes text directly in the GPU in practice.

Think of it like a function that takes text as input, and returns positions as output. Shaders don't really know anything about text. Sure you could probably implement it if you wanted to, but why would you? I think it would add complexity for no benefit (not even performance).