Maybe whites/indians/asians aren't "preferred" in the sense of active racial discrimination. But the people currently at the company, being mostly of that ethnicity, may have personal contact lists/rolodecks that are weighted toward that ethnicity. I can attest to that as a Vietnamese-American myself. The vast majority of my childhood friends are white because I grew up in the Midwest. But I had far more Vietnamese friends than would be statistically expected because the few Vietnamese families in the community just hung out regularly even as they integrated with the rest of the town.

If you grew up not just white, but upper-middle class and above, how many black people, statistically speaking, are you regularly in contact with? Versus someone who grows up lower-income or in the inner city? Being relatively well-off, you have a statistically better chance of getting a better K-12 education, which leads to a better chance at a top-tier school, and then to a top-tier company. You eventually are in a position to make recommendations for hires, and statistically speaking, you know more qualified whites than you do blacks and Hispanics. Statistically speaking, at many companies, recommendations from current employers carry a lot of weight.

Over time, this statistical weighting of life experiences would lead to a mostly white/Asian workforce at a tech company. And no one in this process was ever thinking "No blacks".

So to actually get a workforce that includes a representative number of a disenfranchised minority group, you actively have to seek it out. If you are in Silicon Valley, you have to actively make the decision to fly out to Howard U to do recruiting instead of the much more rational and cost-efficient decision to base recruiting at Stanford and Berkeley.