Because anyone who attempts to take justice into their own hands will have their life destroyed by the incompetent backwards judicial system. The only cases where this kind of reaction should be expected, on an individual level, are cases where someone's child, especially if it's their only child, is killed. At that point a parent's life has already been destroyed and the threat of the law is no hinderance to them doing the needful. Unfortunately, in cases where exactly this has happened, not even such parents stand up an act.
There is no money to be made that way. Gotta drag it out for a year until it falls off thr newscycle and then give the nigger probation. No mass outrage (though lets be honest, there wouldn't be anyway) and the lawyers and courts had a guaranteed income source for a year. Pretty good return on investment for a nigger, probably better than cotton.