It's not like there's a large, untapped pool of talented black software engineers.
I'm at a large tech company. There was a team I worked with for a big with some really talented people, the sort of team where really smart domain experts go. There was a black guy on it who asked questions showing he wasn't an obviously talented domain expert. I found out he was recruited for coming from an unconventional background and was spending time with different teams.
Large tech companies are trying really hard to finds diamonds in the rough, and they're taking chances on people that aren't talented enough.
The other issue is blacks are only 13% of the American population. Seattle is only 7%. Now imagine you're an even smaller minority. It's about people not being assholes, not "hiring more x people" because that will always fall short for some group.