It shouldn't be cheaper than using the space for literally anything fucking useful. Using the massively wasted space to build a whole other apartment complex should vastly outweigh the costs of more efficient parking.

I genuinely think part of the problem is a lack of imagination from developers. Debt to offset the construction costs is a no-brainer if the value of the new housing properties will be above the cost.

The issue is that's complicated and it's easier on their time and brainpower to do something else nice and simple instead of something complicated. It's money being left on the table because they can just do something easier instead.