Well for one, it's cheaper to build strip malls than the urban-friendly buildings that originally populated the main streets of Detroit.
Two, the buildings were also built in a time that Detroit wasn't as car cultured, and had a functioning mass transit system which aleviated the immediate need of convenient parking versus walkability. The businesses must have a low cost option for their customers to access their business so they can spend money there.
Bottom line, Detroit will never be what it was [[not even downtown).
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