Late last year I started a game against Crafty Spire, and after a few hours, hadn't seen any Spirecraft. So as a test, I started a fresh game with full visibility, and noticed several spirecraft around the place, mostly on homeworlds or coreworlds, and largely of the suicidal type (from memory, I think they were Martyrs and Rams). My guess is that the reason I hadn't seen them in my original game is that they were all destroyed fighting minor factions (Human Marauders, Roaming Enclaves, Neinzul Wardens, Dyson Sphere) early on, and as Keith says, the AI doesn't get any more.