According to my understanding, rimliu is proposing that
wage ~ education + experience + gender + race + additional_factors + error
where
additional_factors consists of other unknown factors (e.g. charisma, intelligence, appearance?). In which case, if
education,
experience,
gender and
race are kept constant, there is still going to be a fair bit of variance in
wage caused by these unknown factors.
Either way, it would be pretty hard to model when these factors are far from orthogonal. Imagine the impact race would have on work experience if race is a big factor when hiring people.