Leonardo, Michelangelo, Picasso, Mozart, Cezanne, Bach, Rembrandt, Goya - none of these artists stuck to one instrument or medium, most were masters of many and all experimented freely. Your thesis does not hold true for many great artists I can think of.
Some people that grew up being told that they were smart tend to think of themselves as that kind of master polymath type and end up ignoring that detail.
