I think the main reason why you don't see many buildings with parking ramps integrated into them is that parking structures typically have a life span of around 40 years. Buildings [[if built properly) can have a lifespan of 100+ years with proper maintenance and periodic renovation.
Parking in basements is not as much of an issue unless it is a multilevel underground ramp. A concrete slab on grade can have a long lifespan, and is not as much trouble to replace should it deteriorate.
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