I've heard this idea from multiple places, so I expect it's one of those ideas that is easy to think of but difficult to implement. A similar idea was expressed in the plot of The Music Man, where children are taught to play instruments by imagining that they are whistling.

I wouldn't be surprised if the main implementation problem is that many people are naturally off-key when whistling or humming, and those on-key know the notes well enough that it is more efficient to simply enter music via a piano-like keyboard.