Because that's how some people see it, isn't it? Privilege is an invisible thing that other people believe in. "Sure," they say, "Minorities encounter problems, but so do I. How do they know it has anything to do with race or gender?" The cynics might even believe that "privilege" is an attempt to tip the scales by claiming that they are not already balanced. (See: belief in "reverse racism")
Many privileges are invisible. And they are not only used to justify things like common decency (which should hardly need justification!), but also things like affirmative action. So how can we provide evidence for its existence?
First, I think we should explain why privileges are invisible, and why this is unsurprising. Let's cut through the rhetoric; privileges are not about advantages, but about disadvantages. But if we call it a disadvantage, then it looks like Someone Else's Problem. By reframing it as a privilege, it becomes more about the privileged person, and more relevant to the privileged person. To illustrate, let's look at the first three items from the White Privilege Knapsack.
1. I can if I wish arrange to be in the company of people of my race most of the time.The whole point of the privilege knapsack is to get you to realize... non-white people do not have these privileges. They can't easily arrange to be in the company of people their own race. They have to deal with culturally trained mistrust on a day-to-day basis. They have to worry about housing discrimination, which was legal as recently as 1968, and which may still exist de facto.
2. I can avoid spending time with people whom I was trained to mistrust and who have learned to mistrust my kind or me.
3. If I should need to move, I can be pretty sure of renting or purchasing housing in an area which I can afford and in which I would want to live.
I think privileges are a valid way to reframe disadvantages, but for the purposes of this discussion, it is unhelpful. If you think of them as disadvantages, of course they are invisible to people who do not experience the disadvantages. They are not about your experience, they are about someone else's experience.
Therefore, the problem is not explaining why privilege is invisible. The problem is that the evidence for privilege mostly comes from the personal experiences of underprivileged people. How can those people translate personal experience into objectively acceptable evidence? How can we correct for conflict of interest (since underprivileged people obviously have an interest in demonstrating the existence of privilege), without overshooting?
Though I posed this problem, I will not be able to pose much of a solution.
The obvious way is to point to studies showing real disparities in income and health, like what I did to show gay and lesbian privilege over bisexuals. I'm sure similar studies show a variety of disparities between white and black people, and between men and women, but I will not cite them since my readers are just as good at googling as I am.
Another way is to recount specific experiences where race or gender clearly played a role. But anecdotes have their own slew of evidentiary problems. In any case, it usually isn't clear that race or gender played a role. If, for example, people are talking over you, how do you know it's because you're a woman? All you know is that it seems to happen pretty frequently. This is one of the problems cited by feminists, but it would be hard to produce any sort of "elevator" controversy out of it.
Dear readers, can you think of any ways to provide evidence for invisible privileges?