There's also just way way more aesthetic visual media than there is good music.

And the limitations music tends to hold itself too makes it incredibly hard to make anything original as a whole. In art, you've got a whole canvas you can fill with whatever you want and have quantifiably more ways to express things. When you've only got so many notes you can play on an instrument to experiment with, you'll randomly come across melodies that are extremely similar to things others have already made until you go back and think "hey wait that sounds like something... Oh it's basically the The Office theme"

Every possible 8 or 12 note melody (on a standard scale, anyways) fits in 601 GB. And despite how easy it might be to land on someone else's melody because only a small subset of melodies actually sounds good, some artists have been extremely litigious when it comes to "copying" melodies. Supposedly it's a pretty big problem on the jazz scene too. Plus sampling pieces of other songs tends to only be defendable if you're making a parody (ala lots of Lemon Demon's discography) or reach a certain bar for transformativeness that's much easier to reach in visual mediums. Not to mention that instrument/sfx samples tend to come from big packs that are separately licensed, so if you sample them from another song you may be trampling the copyrights of the instrument pack.

In visual arts, there's much more freedom. In digital, you've got at least 255x255x255 possible colors per pixel, canvases can get massive, and the combination of art styles and subjects are essentially infinite. And if you train a small 4GB AI on hundreds of terabytes of images, it's going to learn how to reproduce common subjects and concepts, styles, etc without physically being able to copy any one copyrightable element. There are outliers where something very exact and specific appears too often in the training data (Getty logo, and arguably the Mona Lisa, characters, for instance), but that doesn't detract from the AI being able to produce what are wholly new pieces of art that don't infringe on copyrights, and I question whether an AI being able to reproduce Micky mouse constitutes as the AI itself infringing copyright rather than the user infringing if they publish said image.

Point is, there's a good legal argument that image generation tends to not infringe copyrights (it lands squarely in fair use because it isn't copying/memorizing training data, barring people using it in stupid ways) while music generation AI could frequently spit out arguably copyrighted stuff due to the sheer numbers difference between training on over a billion unique images vs a much smaller number of songs that have fewer distinct pieces that make them unique (and a much more litigious music industry that wrongfully protects tiny note snippets that probably shouldn't be copyrightable in the first place).

Maybe music AI will get better, but it sounds like a lot more work needs to be done there (or Google's just scared of the MPAA RIAA), and the case law on music copyright would also have to change.