If I'm an employer, why would I hire a dropout - black, white, hispanic, asian, of any ethnicity - when there are other people who are capable of doing the job? This can (and does) lead to underemployment and the issues that we now have to deal with, but from the employer's side I can see where they're coming from. You can hire somebody who (probably) has no experience and didn't finish high school for whatever reason, or you can go with a person who at the very least did finish school. It seems to be an easy decision to make. The real problem here is the educational system and economics that breed environments where people aren't finishing school, or otherwise not being prepared to enter the workforce.
You're right in the sense that the educational and economic system is failing blacks an minorities. Watching The Wire, you see a perfect example of people who are born into violence and drugs and generally a life of welfare where they see rampant police abuse and hopelessness. When you are born into this system, it's really hard to imagine any other life. Dropping out of school for many of the teens born in ghettos is only another part of life.