Not sure of the answer, but I'll think aloud a bit. "Flute" does seem to be our default name for any foreign reedless wind instrument. (Though I'm more used to seeing just "panpipes" and "shakuhachi" alone, without the word "flute" added.) I wonder if it's a way of thinking that goes back to Renaissance and Baroque Western Europe. Back then, there were only two common reedless woodwind types here: the "flute a bec" (which we now call a "recorder" for some reason) and the "transverse flute" (ancestor of the standard Classical flute). In most languages, both had some variant of "flute" in their names. So as Westerners encountered other similar instruments, we extended the "flute" label to cover them.
Europeans of the period knew a much greater variety of stringed instruments, some of them of Middle Eastern origins. Lutes, rebecs, viols, guitars, fiddles, citterns, lyres etc. etc. And there was a habit of coining new names when new variants came along, like the theorbo, which is a modified lute. So that early variety has made our culture less likely to look at a sitar and call it a "guitar".
That's my theory, anyway. I'd be curious to see if anyone's done some actual research into this!
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