They are just crazy
In all honesty, painting (even in digital) definitely helps with this problem. I barely painted before this year (didn't really feel good about my abilities), so doing artworks like this really makes me step away from the usual line art flatting shading combo I've been doing for ages now and strive for less layers. Also makes you relax a bit, even if it looks a bit messy, well, let's just assume it should look like this