Construction time is dominated by staging materials (shipping, delivery, and movement of those materials around on site and into place), and sync points. Houses will sit for weeks waiting on a chain of specialists to get time to finish some critical piece.

In larger projects, there are generally larger staging issues (less footprint to put materials relative to the amount and size of materials), and there are many more specialists involved (including inspectors), increasing the frequency and delays of sync points. The materials for large projects are also much larger now. The Empire State Building exterior was built of bricks, layer by layer. The exterior of most modern buildings are giant panes of glass, requiring cranes and such to move them into place.

I think examining the sync points and staging requirements could shed some light on why construction projects are taking so much longer nowadays.