This is an oversimplification imho. Many of the greatest jazz musicians like Miles Davis and John Coltrane played instruments that could only play one note at a time, and they still manage to convey harmonic concepts.

I don’t have an answer for why most music teachers don’t teach theory (and improv) other than it’s more challenging and they might not know it themselves, and it’s easier to teach rote memorization.

There are plenty of people who learn piano (which obviously doesn’t suffer from the single-note “limitation”) and also learn almost no theory.