Because it is an inside joke of the kommunity. It is intended to be humorous. (Also, KIA could be killed in action, and let's be honest; if your kerbal blows up in a capsule plummeting 100 m/s it stands to reason he's not coming back, unless you have respawn on) EDIT: And like any joke, it gets old after it's been told the 6298th time. EDIT2: And, I hear that before Kerbin had it's name, people used to call it "Kearth", so it's been a part of the community for quite a long time.