Well, some people promote their posts and they have to pay for that. Second, by buying into steam, you can have more influence over what people will see, so there is an interest investing in this that way. Also, although you make money, they know many will keep the money in the platform, so no real money need to get out. Finally, this may be a pyramidal scheme. I'm just a beginner here to but this is how I see it. Welcome to Steemit!