Because they commit more and worse crimes. They are not more likely to be arrested and convicted than a white person. And we know this because of surveys of victims and crime reports that record the race. Like that chart I posted.

The only part of the system that shows any possible racial bias at all is sentencing. And it's only 19%, much smaller than claimed elsewhere. It's well within the margin of error in this stuff as well, given they can't possibly control for every relevant factor (e.g. if their crimes actually are 20% worse. In fact I can't see in this study they even controlled for region, and some states have harsher sentencing than others.)

It's worth noting that ugly people get sentences twice as long as attractive people (but are also not more likely to be convicted.) That's 10 times the disparity observed for blacks. Judges give much harsher sentences before lunch, when they are hungry.

Sentencing is just a shitty system that should be replaced. Some states have tried to replace it with statistical algorithms that predict recidivism. And people like you are the first to protest that.