Because dog breeds are not comparable to genetic differences in humans. They’ve been actively bred for certain characteristics, going through a lot of generations extremely fast in controlled environments in a way that’s impossible in nature. All humans are essentially mongrels. The reason we don’t do racial classification of personality traits in humans isn’t just because it’s morally abhorrent, but also because it doesn’t work, while because of the selective breeding of dogs, there can be big differences in their behaviour due to their breed.
It’s also important that while a dog’s breed does have an influence on it’s behaviour, it’s not absolute, and you can’t actually predict a dog’s behaviour through its breed alone.
Because humans haven’t been selectively bred for specific traits.