"Is this person questioning my credentials because they question everyone's credentials?" "Was that crude comment made because they're just oblivious or because they're actually racist?" "Does this person treat everyone just as poorly, is it something I'm doing, or is it prejudice?"
Having to do that double mental processing can be exhausting if you're in an environment where people are insensitive or outright offensive. Some people handle it better than others but it can take its toll on anyone.
Certainly putting white or Asian would never be a detriment, but probably isn't a positive either. It's just neutral, default. I can't lie and say that I'm white or Asian, so should I just leave it blank? Then I'm hiding something. I must be black. Should I fill it out and say that I'm black, and pray to god that doesn't trigger some internal bias in the split second where someone decides to bin my application or not? Either way it's guaranteed to be a negative or a positive, not a neutral meaningless thing as it should.
That's basically my whole point. It's always going to be either a positive or a negative, not a neutral meaningless thing like it is for white guys. In your experience it's been positive, but I can't imagine that's always the case.