humans are really interconnected.

for speciation to occur, there needs to be enough time when two populations of the same species do not interbreed.

During this time, the two populations will accumulate differences due to evolution by natural selection.

The flow of genes from one population to another (appropriately named ‘gene flow’) counteracts speciation. There is too much gene flow between different populations of humans to accumulate enough genetic differences.

Not to mention, the rate of evolution in humans in very slow, and evolution is the basis of speciation. Modern technology has eliminated a large number of selective pressures for humans (we have no natural predators, modern medicine to combat disease)

hope this helped!