It's because parents incentives come from maslow's hierarchy, not some inherent, always present care for their children, as commonly supposed.
They have, sometimes, a need to feel like 'good parents,' and thus want to do cool dramatic shit, but don't actually gain happiness regularly from long term investment in their children's lives. Thus, 'protect children from evil SEX' is satisfying, but 'invest in child's future long term' is not so, even when the latter child is far less likely to end up in a bad situation sexually than one 'protected' but with no skills, or well developed personality.
The long term solution is to stop parenting as what's basically hobby, and make it into an investment by professionals. Allow corporations or government utilities to own children, and reap a share of their taxes in profit with which to raise the next generation. I hope in my lifetime I'll be part of a family using artificial wombs with a sensible governing structure and specialized roles. IDK what exactly would be the best way to structure it to ensure it's always investing for the future, and giving members pleasant lives. Ideas?>>151226
This is also why eugenics has retarded failures- individuals doing it as a hobby just go 'I am great' rather than systemically working towards improvement.