It’s kind of hard for me to make observations about an era in which I didn’t live, so yes, it’s guesswork, although if your only talking about 20 years ago, not 30-40, my observation is that you’re wrong about their not being more unmarried girls than boys.
A similar observation to the one you make, that you can’t get married without dates, can also be made that a certainl number of girls cannot get married if there are fewer boys in shidduchim than girls.
While a surplus of girls can certainly explain insufficient dates, the opposite cannot be said, that insufficient dates cause a surplus of girls. Your argument ia, in fact, quite irrational.