When someone's culture is sufficiently different from yours, it becomes enough of an unknown to engender fear. And fear due to the unknown leads to paranoid thought, usually at the subconscious level.
For example, if you get a job candidate whose demeanor screams "from the hood", you'll definitely pass him/her over. The last thing you want is for them to start a fight over "respect", or bring some "friends" over to rob the place (deliberate hyperbole, yes, but this is how the subconscious works). Similarly, anyone who has had much contact with Indian culture will know that fraud and corruption are tolerated far more readily than they are in the West. This means that you need to be on your guard lest the person of that culture starts behaving as if in that culture. I've seen it happen enough times in my own experiences, so this is not just idle speculation. This is also why second gen people have a much easier time of it.
You may argue that you shouldn't "sell out" on your culture just to get ahead ("acting white", for example). I would argue that in remaining "true to your roots", you hobble yourself by deliberately refusing to make it easier for the majority to accept you.
I've lived on both sides of this issue. I've seen actually racist people, but far more often I've seen people who are afraid of the cultural divide, and move to protect themselves accordingly. I've also seen people change their own culture to better match the one of the majority, and they made FAR better lives for themselves than those who refused to do so.
Do racism and sexism exist? Of course they do. And in the rare cases where you DO encounter the real thing, it is your duty as a civilized person to put a stop to it. But when it's merely a culture issue, raising a stink about it will do nothing more than stir up resentment.