Why do all big law firms hire ociates before they receive bar exam results?

I keep telling myself, once I get that degree, more opportunities will open up! When that didn't happen, I went to law school and believed that more awaited me after getting a law degree. That didn't open up many job propects either. So I took the bar and thought, "well I have to p the bar to get any law firm's attention." Now I've done that and got licensed. I was only able to interview with one firm, a solo practicioner. He wouldn't talk to me until I ped the bar. When I ped, he said, "let's wait until you get sworn in." When I got sworn in, he offered me $20,000 a year. What an insult! I feel that my efforts were fruitless. I'm wondering how some ociates at major law firms are able to start work there unlicensed.