[pooploop64]

The main reason for them being kittens is to incentivize valuing the astronauts lives without forcing a gameplay related penalty for killing or losing them. The disposability of astronauts is something the devs of this game think was a mistake in KSP. But at the same it's not a game about forcing the player to behave a certain way, so making them kittens is the middle ground.

[TeMPOraL]

What disposability? Kerbals were never disposable! If you crash your little green astronauts on some moon or planet, you're supposed to send a rescue mission after them. And should that rescue fail too, stranding more Kerbals, you just keep launching more rescue missions, until you successfully establish a colony :).

[jmorenoamor]

Rescue missions were one of the funniest and challenging parts of KSP, and a cannonical meme of the community.