Because it's not a vocation, and neither is the priestly state a vocation for that matter.
Source?
Source for the religious state not being a vocation: a book called
Religious Vocation: An Unnecessary Mystery by Fr. Richard Butler, O.P. (I do not remember whether the author is ok with the word "vocation" or not, but he describes it as more of an "invitation". God doesn't
call us to the religious state; rather, God
invites us to the religious state. This invitation is open to more or less everyone who doesn't suffer from an impediment. If you want to become a monk, and if you're able to do, then it's your choice. Do it if you want; don't do it if you don't want. Which to me is not a "vocation". A vocation is not an open invitation to everyone and anyone. A vocation is an exclusive
calling. God appoints you (and only
you) for some particular task, and you don't have a choice. You either joyfully do what God says because He's God, or else you sin by not doing it.)
Source for the priestly state not being a vocation: various SSPX priests, including the vocations director at one of their seminaries, have told me this. (What the priest said is that any unimpeded young man, motivated by holy desires, is free to enter the seminary. It's a personal choice on his part. If you want to go to seminary, do it (no need to be called); and if you don't want to, then don't. But if you do decide to do it, and if you make it all the way through seminary (which doesn't always happen), then the Church will "call" you to be ordained. (A different priest told me that it is the bishop who "calls" you to ordination.) Which I guess is technically a "calling" (and perhaps sometimes comes from God in a remote sense)... but to me this all just sounds like equivocation on the word "vocation". This is radically different from the notion of "God calls you to become a priest; you respond by entering seminary".)
This I think all makes perfect sense, and so I can only conclude that "vocations" generally don't exist. But this important information should be available to young people when they are still in high school. Catholics need to know that it's ok to choose to become a priest or religious. We don't all have to go to college and get married.