Cities tend to already be built up. There are streets, water/gas/electricity/etc. already there, and redeveloping doesn't require 100% of the infrastructure to be run out there and built from scratch.

Expanding the suburban sprawl means going to places where that infrastructure isn't already there, which means adding all of it and adding the burden of maintaining it. And when you then waive the taxes that are meant to pay for the infrastructure, where does the money come from? Out of somebody else's pocket.