Because taking up more land is cheaper than building up thanks in large part to the various rules and regulations of municipalities and states. And in many pla

Because taking up more land is cheaper than building up thanks in large part to the various rules and regulations of municipalities and states. And in many places a parking garage is valued and taxed as a structure whereas a parking lot isn't. Prefab concrete parking garages are a thing the same way that prefab steel warehouse buildings are. The structure isn't that expensive over the life of the structure. It's the latent cost of building and owning a structure that pushes the economics in favor of parking lots. Ironically in a "you are the problem" sort of way, the demographics doing most of the complaining about parking lots and a general lack of density are the same demographics who are in favor of fine grained management and regulation of what people do on their land that causes this situation. You reap what you sow.