Everyone (not only musicians) comes up with musical ideas in their heads. The proof is that you can ask anyone to hum an improvised melody over a chord progression ; and they probably sound nicer than the average musician improvising on his instrument.
Why? Because they don't have to know what notes they're singing. All their attention goes to creating the ideas rather than applying them.
The artistic part in a musician's work is coming up with the idea, the rest is petty much technical. The skills the musician develops (ear training, theory, technique...) to execute (play, write, arrange...) those ideas are purely technical. The only way this effort contributes to our musical imagination and creativity is by exposing us to way more music than the average person. If a child were born with these skills, it would not benefit him in terms of creativity, it would just allow him to express his ideas more easily.
Is harmonizing a melody by relying on 90% theory and 10% ears artistic? Is running mechanically up and down a scale artistic? Is memorizing what scales to play over which chords artistic? Would you think I'm a genius if I used secondary dominants just because I know about them?
Sure all musicians fall back on their knowledge of theory to fill up the gaps. And musicians have better ears and can recognize the sounds they hear (or some of them at least)... But where is the art in that?
I'm not arguing about the importance of music theory. And I'm certainly not arguing about the importance of ear training. My question is: are musicians more creative (musically) than non-musicians (people who only listen to music a lot)?