My best guess is that it is the capacity to invent solutions on spot and think by ourselves, that we are losing. Before emergence of societies, the technological progress was severely limited by information loss. Any smart individual could massively improve fitness of the relevant genes by (re)inventing some basic, but extremely effective techniques, which he'd teach mostly to genetically related individuals. The technique would easy become lost, creating again an opportunity for intelligence to succeed - reinventing it.

Except most discoveries which might confer a decent amount of fitness - enough for fixation to have a chance - are rare. For example, Tasmania had a population of thousands, yet they lacked almost all technology, and couldn't even make fire (instead relying preserving existing flames); if anywhere someone would be inventing technology to help their kins, Tasmania should have seen such secret wiles. But no.

(This is part of a general argument against individual selection for innovation: innovation is too rare, and diffuses too rapidly to unrelated or barely-related people, to be an advantage for the individual - however excellent it is for the group.)