It was because it was in the Great Depression that the builders could hire all the extra talent they needed for cheap.
You could build that building today in in under a year and a half IF you were willing to pay top dollar for all the designers, planners, and construction workers.
You neglect to think about all of the new regulations, the growth of bureaucracy, and the vast number of lawyers now compared to 1930. There is now a vast industry of lawyers tying up projects for years, sometimes for decades.