- Speech appears to be universal to all human beings capable of producing and hearing it, while there have been many cultures and speech communities that lack written communication;
- Speech evolved before human beings invented writing;
- People learn to speak and process spoken languages more easily and much earlier than writing
There are, on the other hand, orthographers who study written systems specifically. Orthography, though, is a rather unpopular subset of linguistics currently; most research is going into things like syntax and language acquisition and less focus is being put on things that are more philological in nature.
In short, I don't pay attention to how people write all that much as a linguist. Ultimately writing is a data-poor fossil of how people wish they could speak. We want data about the type of communication that happens on a split-second basis outside the realm of the grammarian's conventions.