Why is whistling so ignored as a musical instrument?

There are a few recordings of whistled tunes, e.g. by Roger Whittaker. And there are a few songs with whistled sections. But these are few and far between.

You can't whistle words, of course - but a lack of words never harmed the violin, the flute, the piano, etc. etc.

Whistling really is the poor relation of singing! There are just 2 ways that we humans can create melodic music without outside help. Why is one of them so ignored?