It's cheap because there's no demand for it.
Sure, you could buy houses cheaply, demolish them, then wait. But if you do the numbers it probably wouldn't be worth it. There'd be the purchase price, the demolition cost, the continuing taxes and insurance . . . all in hopes that "the area picks up again." It's like people who buy vacant land. Sometimes development moves in their direction and they can sell for a nice profit. Other times, they're just stuck with vacant land that no one else wants, either.
As for hidden catches: Not sure, but I can speculate. In some cities (such as D.C.) the tax on unoccupied property is 10 times the rate for occupied property. Detroit could impose a similar tax. If it did, it REALLLY wouldn't make sense to hold it for 10, 15, or more years. Second, I understand that Detroit is considering buying back some of the land for parks or urban gardens. If so and if it's doing so by power eminent domain, you might end up having to relinquish your property for whatever Detroit said it was worth. And that could be less than you even paid for it. And--though I hope it doesn't happen--what if Detroit never picks up again?
What you might consider, instead, is buying cheap rental property in Detroit that'll cash flow. Then find a property management company so you don't have the headaches of being a landlord. That way, you'll be making some money from Day 1. And if, eventually the city does recover, the value of your properties should rise.
A good Realtor in Detroit can give you more information and advice.
Hope that helps.