It's not traditional leftists that say that. It's shitlibs, which are a different animal. They say that because of 24/7 jew brainwashing that was formulated after ww2 with the frankfurt school and other primarily Jewish social movements. They set about deconstructing European/white identity in order to make the world safer for Jews(in their minds), and allow Jews to dominate globally without whites acting as rivals or getting in the way. Thanks to Jewish money and media influence this school of thought has proliferated to the point people are deluged with it constantly. Much of it subtle and some of it overt.

Eventually some Jews got tired of all this and didnt think it was working well enough thanks to the internet and the free flow of information contradicting their narrative, so they just decided to just push violence, intimidation and dystopian censorship in order to make whites go along with the program, which is how we got the summer of floyd, antifa, deplatforming, etc.