Why is this only a recent phenomenon? Girls historically always entered the shidduch market at a younger age than boys. I can’t explain why but I have a feeling that the age gap isn’t the real problem.
I think it’s the change in attitudes. People were a lot simpler and less demanding of perfection. Boy and girl dated, liked each other and got engaged. Nowadays it’s a ridiculously arduous process with demands and expectations that previous generations would be bemused, puzzled and not to say disgusted if they could see it.