Canon?

[BlackBicycle]

The true story is ...Kerbin isn't their homeworld. Jeb, Bob and the rest are just survivors... ...They just want to get home, really. Desperately...

That is just beautiful. Ever thought of taking up writing, sir?

[Whirligig Girl]

The true story is that Kerbals are actually a hyperadvanced race.

Only the twist is, Kerbin isn't their homeworld. Jeb, Bob and the rest are just survivors from a transteller seedship that malfunctioned and crash-landed in an uncharted system. Only the growing vats for the cloned frozen embryos, the embryos themselves, the mega 3D printer VAB that can manufacture whatever you can think of from base elements, and a broken teach-bot survived the crash.

The poor new Kerbal hatchlings can't learn much from their only teacher, so they grow up without proper knowledge of the world around them. All they can understand is that they are lost and they arrived on a rocketship, so they spend their days vainly attempting to rescue themselves by strapping fuel and engines together with them on top.

They just want to get home, really. Desperately.

This is my theory as to why old Kerbin was called Kearth, it was a different planet in a different system, and the Kerbals finally got home to a planet Kerbin that had suffered some sort of war, so they told them all about rockets, and everyone started to get into the whole rocket thing. It was a great way to rebuild the economy. also, I think that there are cities and animals on Kerbin, we just can't see them because it's a game about flying rockets and spaceships not grand theft auto. After all, Earth has no cities or animals in Orbiter 2010, but it is still the real Earth.