Terran AIs could reproduce the way Peloran cybers did before Contact.  They just didn’t do it much.  AIs were written by humans.  It was the way things worked.  But Contact showed them a new way of living they’d never truly considered.  Peloran cybers reproduced by modifying their own code and creating new cybers.  Sometimes they swapped code with other cybers to create their children.  The Terran AIs became something fundamentally greater when they began reproducing like that.