Well, even if somebody uses a mouse and Photoshop to make their art, they're still holding the mouse with their hand. The output is still a reflection of their skills and creativity. So I prefer to say "traditional" versus "digital," rather than "handmade."
I make my comic digitally. But before I started drawing this comic, I actually wanted to do it on big 18"x24" paper using charcoal. Problem was, big art is a PAIN to scan, and charcoal is extremely messy. Not really an option for me as I live with a hairy dog that's literally bigger than me, and I don't want to deal with the charcoal dust getting on the dog, or the dog destroying the work while I'm not looking.
My other favorite traditional medium is oil paint, but yeeeah that's even more of a pain. Disposal is a ton of hassle, thanks to oil paint/ turpentine being considered hazardous material.
So it's largely a matter of practicality for me. I don't have to clean up after myself after each session; I just hit Save, close the program and put the stylus away. I don't have to deal with storing boxes and boxes of finished art (same reason I prefer to read comics digitally these days). I don't have to worry about my dog getting herself dirty, or worse, hurt because of my art supplies.
Personal preference is also a big factor. You say "why do it digitally" and I say, "why not?" I mean, what are the reasons you do it with watercolor? I'm guessing you like it. Some people like doing it digitally, just the same way you like doing it with watercolor. Different strokes for different people. I genuinely enjoy making art digitally, as much as I enjoy doing it with charcoal or oil paint.