Something to consider here is that selective breeding of dogs is very often not done to the dog’s benefit, and indeed in some breeds makes them less healthy. It’s done for the benefit of their owners, either to do some job or for aesthetics, in a way that would be considered absolutely abhorrent if done to people (indeed, a lot of people think that in dogs it can go too far, especially where breeds with health problems are concerned). Eugenics is at least conceived by its proponents as being for the good of the species in a way that dog breeding is not, it’s just that the easiest way to do that is to control who has children, which isn’t generally considered acceptable for people, and the easiest ways to prevent someone you don’t want to have children from doing so are to kill or forcibly sterilize them, which is even worse. Additionally, it’s tricky to know what traits even are detrimental or beneficial to the species, and the personal biases of whoever is in charge come into play (for instance, a lot of historical examples of eugenics being pushed have been by people who were, well, extremely racist and therefore selecting for things like skin color, which if successful would simply lower genetic diversity, which is counterproductive)

Now, as far as comparing humans to dog breeds (as in, why people find it okay to say “x breed of dog tends to be more aggressive”, but not “x group of people are more aggressive”), it should be considered that humans already have a rather low genetic diversity, as far as species go. If were we like dogs, we would probably all be the same “breed”, so to speak, even people from different ethnic backgrounds (sure, we can tell ourselves apart due to not looking exactly the same, but even two dogs of the same breed don’t look exactly alike, and we’re very attuned to seeing small differences in humans that we might not see in other animals). I’d bet it would probably be possible to eventually selectively breed a bunch of humans to have noticably and consistently different traits beyond just skin and hair color (which while visibly distinct from the outside, don’t really represent very much change), but intentionally engaging in breeding humans like dogs like this would be considered, well, a horrible thing to do, and in any case, isn’t what we currently have nowadays.