Because many slavs are half Asiatic, turkic and finno ugric.
Pure slavs look just like germans, english etccc.. Those that live mostly in westrean ukraine, Belarus and westrean parts of Russia are pure slavs.
west slavs are mix of germanic and slavic look, while south have greater mixture and many are outright turks. While the european slavs look much more diverse since even before their migration to the balkans, the area was very diverse.
Many tribes like the huns, magayers, turks of diffrent tribes and mongols r@p3d and destroyed the slavic gene pool.
People still call us slavs because of the word slave, because of how many of us were enslaved by the turks and arabs.