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My humble and educated guess is, there are many films that are under one hour
as well as feature length.  Simply stating minutes only is one of the ways
the film industry tries to keep rules based on consistency.  It's also easier
for editors, projectionists, etc. to guage a films length in terms of feet
and so forth by knowing total minutes.
 
i.e. a film that runs 129 minutes is 7740 seconds.  Film is projected at 24
frames a second, 16 frames a foot (35 mm) so this film would be 11,610 feet
long.
 
Of course, I am only guessing.
-Joe