I don't think this is true - in my experience almost all guitar players, for example, can read tab and I've played with many who can read scores and sight-read

I don't think this is true - in my experience almost all guitar players, for example, can read tab and I've played with many who can read scores and sight-read to a degree; OTOH, I've only played with very few who were good enough to do everything by ear. For one thing, not being able to read any kind of material (tab / scores / chord progression) means you can't play unknown songs without making a bunch of mistakes. Further, while tab doesn't really convey accurately a piece of music, a score (with all notes written note) can very precisely. There is a huge advantage in being able to sight-read a musical score, and no disadvantage at all by having this ability, so why not learn?