Writing is more than this. Writers can arrange characters in 2d space to convey semantic relationships in equations or among words. You can have multiple degrees of emphasis and italics. You can get a sense of which parts of the equation or prose were written quickly and which were written slowly and carefully. Often it conveys a bit of personality. You can even include small drawings or sketches embedded within the text like Gallelo's illustration of Saturn's rings.

I get that we have to strip out this kind of nuance for technical reasons, but claiming that writing ought to be constrained to only what can be represented in plain text feels like claiming that painting should be constrained to only what can be represented in a bitmap.