The only thing we can do is provide a community that people want to engage in. That's it.
We can't force people to participate. We can't make them edit their questions. All we can do is try our best to make them feel like we can provide good solutions / advice. We do that by making sure the questions we keep are of high quality, that the answers we have are good and that we are kind. Which is exactly what the system is designed to do (and I think does it pretty well).
If questions are bad, they get downvoted / closed / deleted. Community members usually try to post good answers. The best answers receive upvotes and (usually) appear at the top, making it easier to find helpful advice. We also have mods and community members to encourage being kind and will remove rude / offensive content. So the system is working towards that goal of keeping people engaged.
If you are worried about losing interesting questions because they aren't clear / need more information and the OP isn't doing anything, you have a couple options. If the edits are minor, you can try editing the question to get it into good shape so it won't get closed. If a question would require major edits / lots of additions, feel free to ask the question yourself (and make sure it has all the relevant information). This may be a difficult thing to do, since you won't have real life, concrete circumstances to base this off of. You may be able to frame the question in the light of research or something similar. But, again, without a real life situation, it may be hard to craft an answerable question that fits within the site's guidelines and scope, so don't be surprised if your question gets closed also.
TL/DR: we can't make anyone engage us. All we can do is be welcoming and provide a high quality, useful site that makes people want to participate.
