People on the left don't turn it into an ethnicity thing because people on the left, most of them anyway, understand that "white" isn't an ethnicity, it is a socially-constructed supremacist in-group that ethnicities can be added to or subtracted from. The term European isn't an ethnicity either, but a broad and vague conglomeration of various ethnicities. As leftists, we recognize, as anyone readily should, that it was mostly the European ruling classes (though not exclusively - no one would deny for example Imperial Japan's settler colonialist ambitions) that overwhelmed the rest of the world with settler-colonialism. Nothing about that is "turning it into an ethnicity thing." Most leftists are also historical materialists, which absolutely refutes the idea that any ethnicity is imbued with inherent traits of domination, but describes the historical events that led to our current conditions as being entirely a result of the material circumstances of any given place and time.

People on the "left" turn it into an ethnicity thing, because humans have always liked to do that.

No, people have not always "liked to do that" by turning "it into an ethnicity thing" - people have always tended to make in-group and out-group distinctions and carry prejudices of each (usually positive for the former and negative for the latter), but that can be done along any lines of convenience and it can also be intentionally rejected. Leftists explicitly reject and deliberately avoid doing this by examining human development through a materialist lens.