Billions of years ago there was a humanoid race in our galaxy that as they expanded, realized that they were the only sentient beings - at least, that they knew of. I have no idea if they ever encountered people who weren't carbon-based life forms or who weren't humanoid, but it seems from their reaction that they didn't. So they propagated other planets with their DNA in the hopes that even once they died off, there would be people like them in the future, people who wouldn't be lonely.