There was a great article about this in Time magazine several months ago (just google time magazine mormons shadchan marriage). In short it pointed out that the frum community grows by several percent per year. For 100 babies born in 2000, there are roughly 110 babies born in 2003, for example. If men tend to marry women several years younger, then for every 50 men that will turn 21 in 2021, there will be roughly 55 women turning 18. So of course there won’t be “enough” men for all of the women.
The article points out that in the chassidic community, in which men and women are the same age at marriage, there is no shidduch crisis.
This, to me, sounds like the best explanation of what’s going on.