That's partially because to create anything worthwhile in these fields, you often need to spend years honing your craft, ideally before adulthood when other responsibilities demand your time. No matter how much innate talent you have, you can't just pick up a guitar or paintbrush one day and make something that'll impress others. How many great musicians can you name that never even touched an instrument before they turned 18?
If the culture among young people has shifted towards other mediums, then there'll be fewer of them picking up guitars and drumsets to focus their efforts on music, and thus the talent pool and number of worthwhile bands will become smaller. There doesn't have to be perfect overlap between musical and visual artistic talent for this to be true.