Lots of misconception in your question, but the key one is that most people in prison on drug-related charges are in for possession, not trafficking. That’s why these two graphs are presented next to each other. If some people who use drugs (and therefore must also possess them) are white, then statistically, white people should also comprise the majority of the prison population, given that possession is that most common drug-related charge. Except that’s not the case. Not at all. The racial assumptions we tend to make about what drug criminals “look like”, evident even in your question, are probably playing a role here.