>>23323498People find random lizards in rocks quite often. In India this has been explained by the Hindu population as examples of those with bad karma being reincarnated into a much lower level of life.
If you're asking why Graveler becomes Golem, you honestly got me. Why it goes from a rock with arms to a lizard in a rock is a little mind-boggling.
The other "what the hell" drastic transformation Pokemon evolutions at least have kernels of reason in them, like Gyarados and Octillery.