The so-callled "races" of humanity are just minor adaptations to local conditions. If a sentient species spreads all across the world in primitive times, then will evolve variations to adapt to local climate and other conditions.
People who live on or near the equator have black skin and hair to prevent UV damage from sunlight and usually smaller lithe builds for dumping heat. People who in arctic or subarctic regions develop heat trapping translucent skin which also promotes Vitamin-D photosynthesis. They have larger thicker builds to retain heat. Pygmies(M'Butu) are adapted to living in dense tropical rainforest in rough terrain. Most tropical rain forest dwelling people are short. Peoples who live at altitude are barrel chested with much larger lungs. Polynesians are adapted for swimming with a layer of insulating fat under the skin but none in their rock hard muscles underneath. People from the Eurasian steps or descended from them, as well as the Masai in Africa, carry genes to allow them keep digesting lactose as adults, 80% of humanity do not carry that gene and largely past the age of 5, must drink fermented milk.
The variation occur fairly rapidly. Modern humans are only 50,000-100,000 years old and all the variations we see today around in the time when the first modern humans migrated out of Africa (which still retains 80% of human genetic diversity.) IIRC, the lactose gene is believe to be around 8,000-10,000 years old.
If wanted a homogenous species, you need to keep them in a homogenus environment for most there evolutionary history.
Humans migrating into new environments drives the adoptions of variations. The way to create a relatively homogenous looking alien population is to have them evolve to sentience within a single but large and uniform regional biome where the selection pressures will be the same resulting in individual that look the same. As biomes are primarily defined by latitude, which defines climate. A single biome that fairly narrow follows the same latitude for a long distance, will have the same climate and exert the same selection pressures over it's length.
Imagine a planet in an ice age with a large but isolated continent like Eurasia whose Northern areas behind a line of mountains, are covered in a kilometer thick ice sheets leaving only a 500km strip of coastal plain in the south that runs uninterrupted along the line of the equator nearly perfectly for 10km or more. The biome would be uniform because the climate would be uniform for laying in the same latitude. Plants and animals that evolved on one end could easily migrate to the other without any adaptation. So could sentient life forms.
(This true of the central Eurasian Steppes. Plant and animal species vary little from Pacific to the Black Sea.)
All the sentients would have the same markers of adapting to the equatorial zone e.g. if they were humans, everybody would have black skin and hair of the same shade. Since migration of peoples and trade would be easy, genes would flow up and down the coast rapidally, preventing geographical isolation needed to produce variations.
If the sentient species had developed to a sea faring civilization before the ice age ended, then at the end of the ice age, they would spread all over the world but it might be just a few hundred years or so before they became space fairing. Not really enough time for significant variations to arise and in any case, the sea travel would swap to many genes around from place to place to provide the geographical isolation necessary for visible variations to arise.
Such a world would have an overall lack of diversity and relatively few species of all kinds because it started out with just the one biome. When those aliens visited worlds with dozens or hundreds of biomes, they would be startled by the riot of variations of all species.