I was browsing through this list and noticed a lot of buildings were completed between 1930 and 1933. Is this mere coincidence, or is there a connection?
Uneducated guesses from my part:
Its a common thing that large office building such as skyscrapers often are build just before or during a depression, the same was said during the 2007/8 global downturn. http://www.businessinsider.com/skyscraper-index-skyscrapers-signal-financial-recession-2012-1?op=1&IR=T
The reasons for this is mainly your point 2, building buildings takes a long time, sky scrappers especially, the empire state building started in January 1929 (before the wall st crash) but didn't open until 1931.
However why people tend to invest in skyscrapers instead of traditional offices is interesting. One argument is that the fast expanding (and overheating) economy forces up land prices meaning to save costs developers build upwards instead. Another argument is that businesses use skyscrapers as status symbols (the tallest building etc) and in an overheating economy (revenue is still high but expansion has stopped) they have the money to splash out on expensive buildings.