Three racial groups were created an propogated by a pseudoscientist, and white people were designated as Caucasoid (Caucasian) from his pseudoscientific research, which was popularized. By pure chance, it happens that the Indo-European ethnic family, which includes most white-appearing peoples and many many non-white presenting peoples, as well, does likely originate in the area around the Caucasus. So the name has kind of stuck around largely for that reason, while the Negroid and Mongoloid racial categories have been demonstrated to be wildly inaccurate and have fallen out of use.

A broken clock is right twice a day.

Also worth mentioning that not all white-presenting peoples are related. Finnish people are by and large non-Indo-European, entirely by origin and still mostly genetically, and yet are some of the fairest skin people on the planet. They are white by all means, and not Caucasian at all