A bit dark, but I think that all real Kerbals are dead. You know, maybe a plague hit Kerbin, or there was a massive war, or something else entirely. Probably the massive war because there isn't a single building or artificial structure on Kerbin (except for KSC buildings) The entire KSC is, I think, ran by some sort of either superintelligent non-Kerbolar species, or some sort of AI
That's why/how you can instantly get new Kerbals as astronauts. They're mere clones, created by either the aliens or AI. And that's why they're always available, don't care how cramped they are and for how long, don't need food, air, or any life supply. They are built so they don't require them. That's also how you can specify each characteristic when you spawn a kerbal via console menu. It's just the cloner UI