Hmm I've seen someone draw/paint on both paper and an iPad. It didn't look to me that she was much faster on digital. But then it was original art not copies of something.
To be clear: I fully think that digital art is art.
But it is a lot less effort in a lot of ways. If I'm painting in Photoshop or Krita, I have layers. I can adjust blending modes. I can move them up and down. I can easily ctrl+z a mistake. I can rotate my workspace quickly and easily in ways that would be difficult with physical media. I can easily find the color I want without having to purchase more paints and blend them, or colored pencils, or whatever. I can create brushes and variations on them to match my style. I can go on with dozens of other things.
I happen to think all of that is a good thing, and don't know that comparing digital art to AI generated art is a good analogy, but digital art 100% makes many things you do in art easier.
Photographs are a great analogy. If you work to learn about how to take a good photo, and learn all the techniques around framing a shot and how different shutter speeds affect things you can take beautiful pictures. However, if you tried to submit that picture to someone as a painting, they will cry foul. Also most people don't consider your candids on vacation to be in the same "category" as a carefully crafted (or not!!!) photograph of a landscape.
In much the same way, sure you could dedicate your life to learning the craft of prompting an ML model to produce unique, novel, and interesting art, but these communities are saying "we want these to be considered a different class of art" and as mostly self organizing communities they are well within their rights to do so.
Hmm I've seen someone draw/paint on both paper and an iPad. It didn't look to me that she was much faster on digital. But then it was original art not copies of something.