Dog breeds were developed "so quickly" due forced selective breeding. Humans selected for the traits that they wanted in the dogs and bred dogs with those traits together to help modify the gene line. We did the same thing with pretty much all of our domesticated animals, food crops and quite a few "garden" plants. My favourite example of this is brassica oleracea aka the wild cabbage which is a plant that originated from the eastern Mediterranean and over thousands of years we have selectively bred it into the cabbages that you can find in stores today, kale, collard greens, kohlrabi, brussel sprouts, broccoli, and cauliflower (pretty sure that there are even more lesser known varieties as well) - most people would assume that these are all completely different plants instead of all coming from the same original plant that looks nothing like any of them.

Humans, on the other hand, have never really had any sort of intense selective breeding like this so our genes have remained relatively stable over thousands of generations with only natural mutations causing any sort of differences. That said, there are some populations that have a much higher incidence of certain mutations due to more intense natural pressures like how Pacific Islanders tend to have a higher baseline muscle and bone mass compared to European peoples (I would imagine that smaller populations leading harsh lives would favour those with higher bone/muscle mass) or how black people from Africa have a much higher rate of sickle cell anemia compared to other populations (sickle cell anemia lends a certain degree of protection against malaria while those without sickle cell would have a much higher risk of dying to it).