Remembering this one weird theory my friends and I had, Kirby's extra lives are actually just more Kirby's.
In Kirby Super Star's tutorials, Kirby would be on a stage, and in the crowd, there would be even more Kirbs. Further on that, after beating a miniboss or goal game, Kirby would split into three and do the dance rather than any friends joining him. After beating that, I recall getting at least one extra life. In many of the other games, excluding Crystal Shards, the 1-ups were illustrated as a Kirby holding the phrase "1-up", implying that lives are in fact Kirbs. But hey, that's just a theory...