Possibly just anecdotal evidence, but it seems like the pool of good music composers is much much smaller than the pool of good visual artists. Maybe that is an indicator of how "hard" each field is.
Writing somewhat novel music to a formula arguably has a much lower skill bar than producing good representative art (especially if you allow sequencers as composition and disallow basic digital retouching of photos as visual art), but producing something that genuinely stands out may be harder