Why do white people ume that blk africans are dumb when they werent ?

the africans were very intelligent and skilled architects who've built houses out of stone and carved monuments for their most acknowledge leaders in their countries and these achievements shocked european invaders who invaded africa. they thought the black africans didn't have such knowledge to build such magnificent sculptures/buildings. they were so wrong. even that couldn't convince idiotic whitey that invaded africa that africans were intelligent, they enslaved them and the africans pretended to be dumb just to fool europeans into believing they weren't intelligent enough to perform their hard slave labor,they bought into it and also written in many ancient doents how dumb black africans were until they finally realized later on in the process of slavery that the africans played dumb just to fool them as a way to try to escape slavery,but it didn't work. So, why do whites think black africans are dumber than they are,when black africans who come to america succeed and perform a lot better than whites and asians in this country ?