What's in it for them to get other people to believe in gods and souls and spirits. There are plenty of reasons, but that's the other part of the story. Now presume the overactive assigning of agency out to the world is better than an underactive version of it, right? If you're walking around and there's a rock and you happen to think that it has a mind, so be it, but if you're walking around and there's a snake and you don't think it has a mind, you don't think it can attack you, that's probably not a good thing.