My theory is that the older banks needed to impress their clients with stability, conservativeness, safety, responsibility, etc. After FDIC, customers looked to the government for that, and so banks no longer needed to spend the money on the building.
The massive, glittering vaults are sadly gone now, too. The money exists as data on a server somewhere, little need for a vault.
Or an alternate theory: bank owners used to be local businesspeople, who would have been (rightly) embarrassed and ashamed to erect a shoddy and short-lived structure in their hometown. Now banks are at least regional, if not national or global, and care more about saving a few pennies for shareholders than they do about the communities they inhabit.
Combine this with the fact that consumers and "fellow citizens" don't give a crap about good buildings and good urbanism, and here we are with crackerbox drivethrough shite that nobody gives a damn about because it'll be bulldozed in 30 years, after a brief stint as a laser tag venue.
The bulldozed in 30 years attitude is post war nihilism in a nutshell. (In thirty years will be living on mars or dead from nuclear fire)