Like other animals, why don't humans have species & breeds ?
Even my pet dog is one type of breed, but we (humans) lack breeds & species? What is the main reason?
If you with 'humans' refer to our genus Homo (which is often the case), we do have multiple species, see wikipedia for an overview. The difference to many other organism groups is just that all species except Homo sapiens are extinct.
Also, taxonomic ranks below the species level - such as breed, subspecies, population and race - are very poorly definined and are usually used in different ways in different taxonomic groups. It is therefore difficult to draw one-to-one comparisons between these types of ranks across taxonomic groups. It is even relatively meaningless to compare e.g. taxonomic families between taxonomic orders, since they are used in such different ways, and compared to ranks below the species level taxonomic 'order' and 'family' have much clearer definitions.
However, if you with breeds of dogs are referring to distict phenotypes (which is basically what they are), you can easily see that some human populations also have distict phenotypes, so in this sense they are similar.
The reason why the concept of human races is largely obsolete and controversial (along with breeds) is clearly our human collective history of racism, nazism and eugenics.
1) Humans have limited genetic variability.
-This is caused by humans being a new species, only 200,000 year old.
-That went through a genetic bottle neck 50,000-100,000 years ago. Reducing the human population to 3,000–10,000 reproducing individuals
-That only recently managed to expend its range out of Africa ~60,000 year ago.
2)Humans are very mobile.
-The gypsies originate from India. And have been touring Europe for the past 1000 years.
-The population of Madagascar is a mix population of 50% african and 50% South east asia which arrive in madagascar some 1600 year ago.
-Romans... well moving people around europe and africa were their thing too.
3)Humans love making love.
Yup.. you can only get a breed if you have barrier... but there are few barriers that can stop human lust. So our genes get mixed alot. Even when we don't think they should not.
tldr.. our initial genetic variability is limited for several reason. Not enough time has passed to develop that variability. We are also too mobile for distance to be a good barrier... and phenotype change which do arise does not in anyway dissuade humans as a whole from exchanging bodily fluids and gametes.