It's one thing to renovate one home in squatter and abandoned zones for yourself. Another thing entirely to bank on entire subdivisions of people wanting to move into such an area. Who's going to buy the home you build next to the squatters?

And keep in mind, if no one buys and moves in, you're still liable for upkeep on each and every one of the lots. For as long as it takes to sell. Which may never actually happen.