Because they were taught in school or on TV that "we're only one species" and that skin color is the only defining trait. See this Bill Nye propaganda video as example:

https://www.youtube.com/watch?v=78n_FK0CK70

So it manages to stick because only the approved scientists are allowed to speak to millions of people, or write textbooks on these subjects.

When you try and push the opposite (i.e scientists who do prove race is real with evidence) then you get this:

https://www.youtube.com/watch?v=qNshEI6kt80

https://www.foxnews.com/us/california-school-science-project-that-connected-race-and-iq-is-pulled-after-complaints

The entire media dunks on you, or they get you removed/cancelled.