I don't think all artists are leftists, but I do believe that all art that has a broad impact is progressive in character, if not in intent.

Did the Renaissance painters who used single-point-perspective do so in service of seizing the means of production? Obviously not. But they did do so knowing full well that it was a break from the previous art styles, and that such a break was a good thing. Likewise for the rise of the novel, the fall of meter in poetry, digital and electronic art, etc etc

At the end of the day, one of the better (although still incomplete) differentiators between art and simply craft is the deliberate pursuit of novelty of expression. That is, an artist is always trying to say something new, or say something in a new way. You can be exceptionally good at producing the exact same painting, but that doesn't make you an artist, simply a craftsman (and there is nothing wrong with being a craftsman).