I’d frame this a little differently: the vast majority of languages in the world are not regularly written, so linguists must focus on the spoken language. For languages which are written more often, linguists can and do focus on the relationship between the written and the spoken language.
> A language and its written form constitute two separate systems of signs. The sole reason for the existence of the latter is to represent the former. The object of study in linguistics is not a combination of the written word and the spoken word. The spoken word alone constitutes that object. But the written word is so intimately connected with the spoken word it represents that it manages to usurp the principal role. As much or even more importance is given to this representation of the vocal sign as to the vocal sign itself.
Edit: Apparently there's a term for this view: https://en.wikipedia.org/wiki/Phonocentrism
Interesting personal anecdote: My son is learning French in school, he's native in both English and Turkish, but not very good with Turkish spelling and does not use it often. I've seen him struggle with memorizing French words and their pronunciation, because he doesn't have an alternative phonetic representation of them (he doesn't know IPA, naturally :-). But Turkish spelling is (almost 100%) phonetic and I remembered that's how I memorized English when I was kid, I would think about their pronunciation in Turkish! E.g. cheese would be "çiyz".