Honestly? I think because people just give cursory glances at the kids and don't stare at them nearly as much as we (the parents) do.
Also... how old is your kid? When DD was a baby she was the spitting image of DH (and trust me I was really trying to see something of myself in her and got nothing...). Now 1.5 years later I do see some of myself there but her coloration still screams Latin American and so people think of DH first, not me. Sure, if they look then at her lips, her cheeks, etc they can see part of me there too but then they really do have to look at her and not just glance. But we have started getting a lot of comments about how DD looks more like me now. Then I again, I use to get comments growing up about how much I looked like my stepmom, so I take all comments like this with a grain of salt.
And on the other, other hand, I also had one lady assume I was the nanny, so you just can't win.
And, FWIW, my best friend from HS. She was mixed but she was the spitting image of her white mother but had her father's coloration. I mean, nobody, and really nobody could say otherwise. As far as I know she never got comments that said she looked like her dad (because she seriously didn't... at all!).