Good question!
First, that artist will only learn from a few handful of artists instead of every artist's entire field of work all at the same time. They will also eventually develop their own unique style and voice--the art they make will reflect their own views in some fashion, instead of being a poor facsimile of someone else's work.
Second, mimicking the style of other artists is a generally poor way of learning how to draw. Just leaping straight into mimicry doesn't really teach you any of the fundamentals like perspective, color theory, shading, anatomy, etc. Mimicking an artist that draws lots of side profiles of animals in neutral lighting might teach you how to draw a side profile of a rabbit, but you'll be fucked the instant you try to draw that same rabbit from the front, or if you want to draw a rabbit at sunset. There's a reason why artists do so many drawings of random shit like cones casting a shadow, or a mannequin doll doing a ballet pose, and it ain't because they find the subject interesting.
Third, an artist spends anywhere from dozens to hundreds of hours practicing. Even if someone sets out expressly to mimic someone else's style, teaches themselves the fundamentals, it's still months and years of hard work and practice, and a constant cycle of self-improvement, critique, and study. This applies to every artist, regardless of how naturally talented or gifted they are.
Fourth, there's a sort of natural bottleneck in how much art that artist can produce. The quality of a given piece of art scales roughly linearly with the time the artist spends on it, and even artists that specialize in speed painting can only produce maybe a dozen pieces of art a day, and that kind of pace is simply not sustainable for any length of time. So even in the least charitable scenario, where a hypothetical person explicitly sets out to mimic a popular artist's style in order to leech off their success, it's extremely difficult for the mimic to produce enough output to truly threaten their victim's livelihood. In comparison, an AI can churn out dozens or hundreds of images in a day, easily drowning out the artist's output.
And one last, very important point: artists who trace other people's artwork and upload the traced art as their own are almost universally reviled in the art community. Getting caught tracing art is an almost guaranteed way to get yourself blacklisted from every art community and banned from every major art website I know of, especially if you're claiming it's your own original work. The only way it's even mildly acceptable is if the tracer explicitly says "this is traced artwork for practice, here's a link to the original piece, the artist gave full permission for me to post this." Every other creative community writing and music takes a similarly dim views of plagiarism, though it's much harder to prove outright than with art. Given this, why should the art community treat someone differently just because they laundered their plagiarism with some vector multiplication?
Just because something is transformative doesn't mean that it's fair use. There's three other factors, including the nature of the work you copied, the amount of the copyrighted work taken for the use, and the effect on the market. There's no way in hell I believe that anyone can plausibly say with a straight face "I'm taking literally all of the creative works you've ever produced and using them to create a product designed to directly compete with you and put you out of business, and this qualifies as a fair use" and I would be shocked if any judge in any court heard that argument without laughing the poor lawyer making it out of the court.