There are houses that were abandoned 20 years ago and need to be torn down. Then there are houses that have been abandoned in the last 3-5 years, that are salvagable.
The problem is this: crime at the property sites. When I still lived in the city, in the 90s, Habitat built a bunch of homes in northern Highland Park. No one ever moved into the homes because thieves broke into them and stripped them bare of plumbing, electrical wire and anything else they could rip out.
Those who rehab houses in the cities of Detroit, Highland Park and Hamtramck cannot leave their building supplies on site, and have to take them with them when they leave each day. They also have to move people in as soon as they complete their work, or risk people breaking in and taking everything. When they build large projects, like condos or apartments, they hire security to be on site 24/7.