The first and biggest is lack of proper study. Meaning, no study from life, and no study from the masters. Of the two, study from life is the more important one, but both are important if you want to improve.
Diligent, detailed study of real life helps you learn what things actually look like in the real world; this is harder than it sounds. Right now, picture the door to your bedroom. Got it? Good. Now, how detailed was your imagining? Did you remember any architectural flourishes on the door itself, the whorls of the wood grain or the scratches, the style of handle, any designs or scratches on the handle, overpaint on the door latch, that dirty glue residue from that stupid decal you put there years ago and then peeled off? Things are always more complicated than we remember them. This is why you learn from life.
But because things are so complex that they become hard to paint. This is where studying masters comes in. By studying other, more proficient artists, you can learn methods and shortcuts to simplify things down so you can actually render them. If I asked you to make me a painting of your bedroom door, maybe you don't need to intricately detail every single whorl of the wood grain; maybe just a little paint of the right color applied with the right brush will give the *illusion* of wood grain.
This tug between lifelike realism and simplification can be called "style". By studying masters, you learn different styles and how to implement them in an aesthetically pleasing manner.
Next failing of what we might call the SJW artist is lack of proper foundation. Unfortunately, many art schools and programs are glorified babysitters. After all, the student is the customer, and don't you know that art is subjective, man? So many art programs grade the student by the student's own subjective improvement, not by hitting certain external milestones in terms of technique. In short, all too often an art degree is a giant "You Tried!" sticker that sets you back about 100 grand. This is bad because it doesn't teach healthy or constructive art habits, and in art, your habits become your skill. I have seen better art come out of self-taught idiots on the internet than come out of serious art students, because those self-taught idiots are the ones actually putting in the effort to improve and setting healthy art goals for themselves.
And the final issue is the fact that they're addicted to social media. Social media is widely regarded as a necessary evil for artists. But Twitter and Instagram, the two biggest places to get discovered as an artist these days, are both shit. They prioritize new over old content, leaving many artists feeling like they have to keep producing, often at the detriment of the work they put out. This quantity over quality approach can rapidly degrade your skills if you don't make a habit of setting aside time to deliberately improve them.
And that's before we consider the fact that social media is full of, let's face it, children. And children have very simplistic tastes in art, and not all of it looks especially pleasing to an adult eye. Consider the very saturated, almost garish colors of you average children's cartoon. Remember how we mentioned studying from masters? Well, if your "masters" are all children's media, you're naturally going to degrade down to the child's level. (If you're now wondering how cartoonists and children's book artists manage to maintain their skills, well, they're usually doing life drawing in their spare time).
So what we have are a group of artists who aren't learning how things actually look, aren't trying to learn tested methods from established experts, don't have a solid foundation in their skills, never developed good habits, are incentivized to make lots of mediocre art over fewer pieces of higher quality, and have surrounded themselves with art of a technically lower quality.