Maybe NOW they are, but from a bit I've read, it appears that during the 1940s-1960s, with Cold War tensions growing, the government might have kept a close eye on certain people who wanted to travel abroad, and consequently required the passport applications to be scrutinized closely, and renewed at closer intervals. The playwright Arthur Miller reportedly had problems with passport renewal, and I believe Richard Wright did also. So for that period, a passport that required review and renewal more often would seem to make sense.
http://www.thecrimson.com/article/1999/5/12/miller-tells-of-crucible-origins-parthur/
http://entertainment.timesonline.co.uk/tol/arts_and_entertainment/the_tls/article4112123.ece
Passports in the 1960’s or so were good for 3 years and could be renewed for an additional 2 years, There is block in the passport to show it had been renewed (or not). Those passports had an issue date in them but no expiration date but an Aviso below it:
In the passport it says (with an arrow pointing to it) IMPORTANT Unless Otherwise Limited
This Passport Expires Three Years From Issue Date
If Renewed, It Expires Five Years From Issue Date.
a note it also had a place to mark wife and minors.