I work for a place that designs and manufactures parts for a lot of these NYC projects. There are several factors that slow them down, and every step of the process is horribly inefficient. We start the bid and design process years before groundbreaking because it takes so long to get reviews and approvals. Permitting, planning, and impact studies that should take a few months drag on for years. The land prices are so high and putting lots together so difficult that they build tall skinny buildings. Tall skinny buildings take longer to build than shorter, wider buildings. Everything is so hush hush in the preconstruction phase that we don't even know who exactly is building it or where it's at until a few months before they start demo on the existing structure. Shipping and delivery is a logistics nightmare downtown and crane time is always at a premium. Imagine how hard it is to get around NYC anyway, and then imagine how many hundreds of tons of material have to be delivered 24 hours a day by big trucks that aren't as quick and maneuverable as a car. The whole process is just years of waiting followed by half a decade of absolute madness trying to get it all crammed into a city block.