Because no text is understood just by reading the words. — Agustino
So if I write "The cat is black and was a year old when she had kittens" you would not be able to understand it just by reading the text?
Of course your brain would understand it once it was processed. It means exactly what it says.
So when it says in the bible that "Mary was a virgin when she had a baby" your brain would process it and it too would mean exactly what it says.
So why should everything else be a mystery that has to be unraveled before it can be understood?
So was fulfilled what was spoken through the prophet: “I will open My mouth in parables; I will utter things hidden since the foundation of the world.” — Matthew 13:35
parable; a simple story used to illustrate a moral or spiritual lesson, as told by Jesus in the Gospels
10 The disciples came to him and asked, “Why do you speak to the people in parables?”
11He replied, “Because the knowledge of the secrets of the kingdom of heaven has been given to you, but not to them.
12 Whoever has will be given more, and they will have an abundance. Whoever does not have, even what they have will be taken from them.
13 This is why I speak to them in parables:
“Though seeing, they do not see;
though hearing, they do not hear or understand.
14 In them is fulfilled the prophecy of Isaiah:
“‘You will be ever hearing but never understanding;
you will be ever seeing but never perceiving.
15 For this people’s heart has become calloused;
they hardly hear with their ears,
and they have closed their eyes.
Otherwise they might see with their eyes,
hear with their ears,
understand with their hearts
and turn, and I would heal them.’
16 But blessed are your eyes because they see, and your ears because they hear.
17 For truly I tell you, many prophets and righteous people longed to see what you see but did not see it, and to hear what you hear but did not hear it.
18 “Listen then to what the parable of the sower means:
And he goes on to explain.
Keeping things in context helps understanding.
Jesus was talking about things that ordinary everyday people might not understand. Complicated explanations would not have worked so he used parables to place the ideas into to day to day situations that the people could relate to.
But did he ever transmit any super secret information through them? He used parables to make sure that nothing was hidden.