Either ignorance or malice
Ignorance being the case of "the blind leading the blind."
Anonymity means that when someone gives critique, we can't look at their work and determine whether or not they know what they're talking about. I would gladly take advice from my idols, but not someone who started drawing today. Not to mention if someone sounds confident enough, you'll more easily fall for their bullshit.
Malice, because misery loves company. If someone here has "failed" to improve over x years and had to give up on their dream and get a normal job, it is comforting for that person to see other people suffering the same as them. Misery loves company. It just feels good knowing that you're not the only fuck up. That's why everyone wants their friends to succeed in life, but not too much. If their friend becomes better than them, they feel bad, and thus, try to pull the friend back down. If that doesn't work, they tell themselves that the friend changed, or they went hollywood, or they were always a horrible person. I think I wrote too much