Because as often, the truth is in-between, but in this case closer to the replacement model.
Humans migrated out of Africa in several waves over a period of 2 million years, for example, when periodic drought turn Sahara forest into a desert. So from earliest H. ergaster/erectus 1.8 million years ago to the last massive migration 60,000 years ago colloquially called Out-of-Africa.
- Multiregional model states that OoA populations spread over Eurasia, mixed with local previously migrated populations ("natives"): Erectus in Asia, Neanderthals in Europe etc. The various admixtures made recognizable regional features ("human races").
This model is also the Chinese nationalist storytelling: "Chineses are the descendant of Peking Man (a H. erectus), Gweilos (non-Chinese people) are not"
- Replacement model states that OoA populations spread over Eurasia, drove local populations to extinction and eventually replaced them.
Regional features ("human races") diverged afterward.
Molecular analyses show that every human out of Africa has inherited 2 to 6 % DNA of one or another archaic human species.
That means that OoA migrants rapidly outnumbered and almost replaced the natives, but took time to interbreed a little with them (~5 % instead of 50 % for a perfect mix)