Where are you getting that should from? The base took a long time and was literally built like a bunker, but the actual floors went up at roughly 1 floor a week. However, there was a significant gap between all the glass on exterior being up and the building being ready for occupancy.

Part of this is tall buildings simply run into issues with workforce sizes. The same happens for developers building thousands of single family homes run into similar issues. They can build individual homes vastly faster than they can finish a large subdivision.

Elevators are also a bottleneck etc.