Based on the two incidents I've witnessed first hand of this attitude - once when artists were salty that the Inktober guy said they'd only be sharing works on the official Inktober account that used physical ink (from ballpoint pens to India ink) since he felt digital work didn't reflect what participants were supposed to learn from the exercise, and again when Butch Hartman made a generic "digital art is great but it's still important to learn traditional skills" positivity post that everyone got their panties in a twist over - and the two most common trains of thought I saw being against this were:
- Praising traditional art is "gatekeeping" because you're supposedly putting a barrier of entry into art by expecting people to purchase art supplies despite good digital arguably being a greater financial investment between the computer, tablet, and/or handheld devices like ipads.
- People who practice traditional art are backwards dinosaurs who just don't like that digital tools with features like the undo button and layers have rendered them obsolete, and traditional art therefore provides no learning value whatsoever
What's often overlooked however is that traditional art doesn't just teach you skills like life drawing or color theory, it's also very important for learning about the more subconscious aspects of art like posture and hand positioning. You learn quicker with traditional art, for instance, the importance of not letting the side of your hand drag over your page and to reposition yourself or your sheet because doing otherwise smudges your work. It's also important in developing a faster drawing/painting pace while maintaining coordination because you often have a limited number of attempts to get something right before you have to start over. All of these things translate into improving your digital art because while things like transform tools and Ctrl+Z are undoubtedly useful, it's much more efficient in the long run if you hardly have to use them at all.
Another way to look at it is imagining of traditional art was a bicycle and digital art was a motorcycle. Between the two the motorcycle is the more advanced tool and provides more powerful/efficient features that you can't necessarily get from a bicycle, but it would be crazy to hop straight on a motorcycle if you've never ridden a bicycle before since the bicycle helps you with the most basic of concepts that are still applicable to the motorcycle.
Tumblr artists are often the people who declare the bicycle to be obsolete with no useful skills to offer and then proceed to putt-putt away going 2 mph on a $2000 Wacom motorcycle because they have none of the skills to use it properly.