User blog comment:Cory Jaynes/Reapers origins/@comment-1997251-20110805125031

This is all covered in the game. EDI says that the Reapers 'likely attempted to create a Prothean Reaper, but failed.' They couldn't create a Reaper using Prothean genetic material, so they re-purposed them in order to avoid wasting the resource. The Collectors served the Reapers by gathering test subjects and testing their potential viability to create new Reapers. This, of course is the reason they bargain for 'samples' from pirates and slavers. There are no Prothean Reapers, which is why the Collectors exist.