There's no deep seated reasoning. It's just numbers. White women outnumber black men 5:1. Get rid of the racists and you're still looking at 3:1. Just off pure luck and being surrounded by white women, I'm surprised it's still relatively uncommon...especially compared to Asians.
White men don't do it because they know their kids would essentially be "black." As much as they complain, they would never trade places with us and know raising a black son is a completely different experience. A black man raising biracial kids is still raising children that will be socialized as black or at least "black adjacent." Much less steep learning curve.
Also keep in mind most of us aren't exactly 100% anyway. Dave Chappelle is 25% white. Don Cheadle is a deep complexioned breh and he's damn near 20%. We already mixed. That shyt happened.
Gotta be careful though. Cacs won't make us honorary white people like some Latinos or Asians. They just wanna breed us out. Problem is that they will need to almost breed themselves out of existence for it to work.