AZ:
They don’t start out with a “vast” difference, only about 10-12% (about 3% increase in population each successive, younger “grade” is the number I’ve heard).
Since most boys get married, and rather quickly, as you say, the ratio of girls to boys increases as more people from that age group get married.
For example, if there were 100 boys starting in shidduchim and 90 girls, and within two years, 90 boys got married, that would leave 10 single boys and 20 single girls, a much higher ratio than it started with. The numbers are actually much bigger than that, and that’s just from one year’s sample.
The result is that despite the fact that most boys get married quickly, there are, unfortunately, plenty of older single boys, and by this time, a vastly larger number of girls.
Among these older singles, many have difficulty getting married, some because of the fact that they are burnt out, and some for inherent reasons, which is why they became older singles in the first place. Whatever the reason, any boy who doesn’t get married exacerbates the problem, as Oomis said.