They believe that, for instance, intelligence simply cannot be mostly genetic. It can't be. If that was true, it would lead us to all sorts of uncomfortable conclusions about why certain things are as they are. Therefore it is not true, amen.
Interestingly, though, when some of those same folks adopt a child, they make sure to learn about the characteristics of the birth parents of the child, including things that would at some level be indicative of intelligence.