I've already explained why, theoretically and empirically.
(1) First of all, _capitalists as a class extract surplus from workers_; in contrast, whites, "as a group," can't be said to extract surplus from blacks "as a group"; men, "as a group," can't be said to extract surplus from women "as a group"; etc. White workers don't extract surplus from their black co-workers who neither work for nor get paid by them; their employers extract surplus from both, which is the material ground of an argument for cross-racial, anti-capitalist solidarity. *If white workers extracted surplus from black workers*, there would be no such material ground for cross-racial solidarity; *if white workers extracted surplus from black workers*, in fact, white workers might as well go for white supremacy, & black workers for reactionary black nationalism, *which would be in their respective interests*. That's not the case, however.
(2) Whites, _on the average_, gain higher wages than blacks, even if we compare them within _the same occupational categories_, but it is _not_ true that all whites individually gain higher wages than all blacks. White day laborers earn less than black judges, doctors, lawyers, tenured professors, etc., for example. Male day laborers earn less than female judges, doctors, etc. White workers can be & are sometimes exploited by capitalists of color, male workers by female capitalists, etc. also.
(3) White workers do gain relative higher average wages than black workers do. However, more racism makes for lesser wages, social programs, etc. for _white workers as well as black workers_ than less racism does, to say nothing of what white workers might gain in the absence of racism. The same goes for sexism, heterosexism, etc.
Do you have any serious argument against (1), (2), & (3)? Do you actually believe that racism is in the interest of white workers, sexism is in the interest of male workers, etc.?
Those who think that the contradiction between capital and labor is *the same* as contradictions between whites and blacks, men and women, etc. must conclude that racism is in the interest of white workers, sexism is in the interest of male workers, etc., which is basically the argument of "identity politicians." In that case, though, what's the point of opposing the Nation of Islam & the like outfits?
Yoshie