Also because they know it's a lie, but it's been a tremendously successful lie.

I think it has more to do with BBC and other MSM being big on idpol. Mainstream discourse considers idpol as leftism. Just look at what "progressive" DemocracyNow! and Goodman think of Kamala the cop, they are giddy over a black face in power, like they did for Obama

https://www.democracynow.org/2020/8/13/kamala_harris_stands_on_the_shoulders

DemocracyNow! is a big gateway for angry libs moving left, it's no wonder the US left is in such bad shape.