Maybe because they earnt their position through merit? Maybe because white males are still a majority of the work force in Google, Facebook, and may more? The probability of picking 3 random senior people who are white are not that slim.
I as an Asian man take issue that just because you see white people in the show and automatically assume what the show has to say is not representative of whats happening in the Ad-industry. You could literally replace any color of race and claim it is not representative and disregard the bigger message here. This is a classic reverse-racism.