Slums are due to insecure property rights, basically. You can't sell the land if you don't have anything approaching formal title to it, and for the same reason

Slums are due to insecure property rights, basically. You can't sell the land if you don't have anything approaching formal title to it, and for the same reason you aren't going to invest in high-quality housing or amenities. (Note that "property rights" need not be understood in a conventional sense! Indeed, long-term leases are far preferable to simple property in land, precisely because they do get rid of some residual uncertainty and market friction.)