Where does Scripture say that Mary was always a virgin?
If it’s so important, why didn’t the Catholic church include that in the Bible when they *wrote* it?
The Bible says the improtant thing: that Jesus was born of a virgin. The rest is no longer matter for the Gospel, but for the lives of saints. The Bible does not tell us a whole lot about any of the Apostles and Jesus's family. We don't know when Joseph died; how St. Paul's final journey to Rome ended (he was beheaded), how St Peter died (crucified upside down), how Our Lady died and when (no tomb ever existed). This was all the historical knowledge fo the Church that the Church in her wisdom decided not to put in the Bible.
The Apostles never intended to make the Gospels an encyclopedia of Christianity. It is clear from the manner in which they are written. St. Luke, for example (the physician who evidently interviewed our Lady for his gospel) says that he decided "to write to thee in order, most excellent Theophilus, That thou mayest know the verity of those words in which thou hast been instructed" (Lk 1). Note the purpose: not to instruct, but to provide a witness to the instruction. The Church instructs. The Evangelist says "Yes. I was there". The Scripture is there to confirm the body of knowledge already present in the Church. It is not that body in itself.
How do you KNOW for sure that Mary was always a virgin anyway?
This is a knowledge that the Church retained from testimonies of people who attended to Our Lady. It is possible that she allowed herself to be examined, because naturally it was a major contention at the time: Was Jesus or was He not, born miraculously?