It makes me remember the trend where everyone is trying to be SakimiChan. It is not only digital art or digital painting IMO, remember when our Instagram feeds get flooded by prismacolor rainbow lips? Or when Meyoco's simplistic cute and aesthetically pleasing art got booming you would see six similar art on your feed all by different person.
I think one of the cause is because social media trend. One art style exploded or get trendy, so it and many other similar styles get boosted by the algorithm based on audience preference. Then there are people who try to ride the wave too, or changed their art style to something they think as more appealing, adding more to the trend. That might be why at some certain time, I think everyone is like drawing the same thing/using the same style/following the same aesthetic.