Most of these people do not intend to get married. Some of these parents are fallen-away Catholics.
Vocations come from parents who are practicing Catholics, although many are now coming from converted men, whose families are completely secular. However, the garden of vocations has traditionally been the good, Catholic family under the leadership of a dad who understands the meaning of "domestic church."
Where there are strong dads and strong priests, vocations follow.
But, there are other "fathers" who inspire men to become priests. I know for sure that in my son's life, several priests helped him on his way to the seminary-by example.
The first was our pastor who is still working, although in retirement age, who is a trad priest, and one who says the Latin Mass beautifully. He is not only a model priest liturgically, but a father to all he meets.
The second is also a traditional Latin Mass priest of the FSSPs. He was very young and a new priest when my son met him, and this priest was a great role model.
The third priest is a Byzantine married priest, one of the holiest men I know. He is not only a holy man, who has a servant's heart, but also completely orthodox, and liturgically sound.
The fourth priest is a monk with a brain the size of a planet. He was a philosopher teacher, now retired, who was also one of the most honest priests I have ever known. My son greatly benefited from his intellectual soundness, (he taught Aquinas, among others), but also from his fearlessness is standing up to evil.
So, one reason why there is a lack of vocations is that many young men never meet holy, orthodox, inspiring, honest priests, who are truly fathers to their congregations.
Sadly, such good men are hard to find....
My son has been blessed in his life to be influenced by such good men.