Why do people but into the media about mixed race people supposedly having an advantage?

Right. The alleged advantages of miscegenation don't really exist. There's something called hybrid vigor, but you don't need to cross races to get it. Crossing two persons from purified family strains within the same race will give you all the hybrid vigor you desire, without also loading the offspring with as many Mendellian mismatches as you'd probably get from a race-cross.