This is my personal experience. There are three explanations.
1.
White people have better connections. It's easy to get a job when your dad is golf buddies with the tech company manager and he can pull some strings for "Sally" so she can get into a really nice internship. In college, almost all of my white classmates came from professional/prominent families.(
Parents were engineers, doctors, dentist, BSN nurses, computer scientist, military officers, hospital administrators, successful business folk, scientist, bankers, top performing HS teachers in difficult subjects, etc) A lot of their extended families were middle class professionals as well and it wasn't unusual to have a brother or an uncle who was a doctor. It's easy to go through Physics II like it's cake when mom is able to funnel your private tutor 10 dollars an hour for private sessions or get really strong math skills because your mom is the AP calculus teacher at the local HS. When you grow up in the best schools in the state, it's very hard NOT to do well in school. A lot of the nice high paying jobs are gained through CONNECTIONS and NETWORKING, not by slamming out a bunch of random resumes to random people. This puts white people at an advantage because they will be connected to these "in crowd" people need to fill positions. Black people typically don't hang out with these people.
2.
The persistent belief that "black people aren't intelligent enough/passionate enough to do the work so why invest in them?" I'm a bit nerdy but one of the down sides about STEM is that it is filled with nerds and nerds can be some of the most racist/insecure/sexist people on the planet. It didn't help growing up when I told some of my white teachers I wanted to be a scientist and they were like

. I got lucky I had more progressive minded teachers that didn't think black people were too stupid to be high achievers that really motivated me to dream big, work hard and think differently. Then some white professionals are very intimidated by black culture and feel best to hire mostly white people since they fit into "company culture".
3.
There is a lack of black HIGHLY successful entrepreneurs outside of music/sports. Yeah there are black people that do marginally well outside of those two industries but there is a strong lack of
household name level successful black people in the STEM industry. You have Mark Zuckerberg, Steve Jobs, Bill Gates, etc and almost all of the top lauded people are white males.
They typically are going to associate with white males in their inner circle and the people they mentor will often be white males. There are few black zuckerbergs, steve jobs, etc that are more likely going to mentor black youth to have the skills necessary to compete. This poses a huge problem for the "achievement gap".
That's generally now why I don't support black self segregation. You definitely need to be conscious as a black person of how your race is perceived in the work but self segregating yourself is only going to make things worse. This is why I am so hard on black militants on the Coli. Just because you're not a militant doesn't mean that you are a c00n but hating white people is not a great strategy for long term success. There were a lot of jobs, opportunities and skills I learned BECAUSE I regularly kick rocks with white boys.(along with black folks, asians, indians, hispanics as well too) It's important for black people to have their own strong community but not at the expense of self-segregating yourself.