I believe they can make money on a few ways. First, it seems that when you post made on any particular​ gateway site to the steem chain, posts share the reward with that site. Make a post on chainbb or the esteem app, 5% of the post reward goes to chainbb or eateem.

Second, some of these developers work as a witness and ask for witness votes.

Third steem as crowdfunding, you can see that a bunch of these projects get huge upvotes when they make update posts on the steem chain.