Because they are intimidated?They think they aren't good enough?They fear rejection?
Or they think they are on to a sure fire thing?
I have to say I never noticed this phenomenon.Quite the reverse in fact.

but that was far away and long ago.
Maybe men in general have grown up and matured? Finally.
EDIT:Well now I've just seen Russ's posting which rather undermined my attempt at being generous to men...