I (highly trained and experienced musician) reject the premise. I find it a little more difficult to whistle in tune than to sing; in both cases, most of the difficulty is in knowing where the pitch is, rather than in making the muscles produce it. Of course practice is everything. (And how, in these days, did a paper get published with the word "poor" in its title?!)

Poor neuro-motor tuning of the human larynx: A comparison of sung and whistled pitch imitation

"[...]while participants who sung more precisely also whistled more precisely, sung imitations were less precise than whistled imitations."