To be fair, the star patterns do have vague shapes that inspire their name (in Orion the shape of a man with a club and a shield is quite recognizable). But of course pareidolia was applied out of necessity as you say; it was mnemonics probably as much as a good bit of storytelling fun. I would also add another possible mechanism: passing through multiple cultures. Maybe you inherited the constellation name from a previous culture, and mapped their God or hero of choice with one of yours because that makes it more familiar. But maybe some part of the analogy was lost in the mapping. Do it enough times and name and constellation may seem now associated in a completely arbitrary way.
To be fair, the star patterns do have vague shapes that inspire their name (in Orion the shape of a man with a club and a shield is quite recognizable). But of course pareidolia was applied out of necessity as you say; it was mnemonics probably as much as a good bit of storytelling fun. I would also add another possible mechanism: passing through multiple cultures. Maybe you inherited the constellation name from a previous culture, and mapped their God or hero of choice with one of yours because that makes it more familiar. But maybe some part of the analogy was lost in the mapping. Do it enough times and name and constellation may seem now associated in a completely arbitrary way.