Chesterton notes this assumption in the "Ball and the Cross"
With a smart journalistic instinct characteristic of all his
school, the editor of The Atheist had put first in his paper
and most prominently in his window an article called "The
Mesopotamian Mythology and its Effects on Syriac Folk Lore." Mr.
Evan MacIan began to read this quite idly, as he would have read
a public statement beginning with a young girl dying in Brighton
and ending with Bile Beans. He received the very considerable
amount of information accumulated by the author with that tired
clearness of the mind which children have on heavy summer
afternoons--that tired clearness which leads them to go on asking
questions long after they have lost interest in the subject and
are as bored as their nurse.
The streets were full of people and
empty of adventures. He might as well know about the gods of
Mesopotamia as not; so he flattened his long, lean face against
the dim bleak pane of the window and read all there was to read
about Mesopotamian gods. He read how the Mesopotamians had a god
named Sho (sometimes pronounced Ji), and that he was described as
being very powerful, a striking similarity to some expressions
about Jahveh, who is also described as having power. Evan had
never heard of Jahveh in his life, and imagining him to be some
other Mesopotamian idol, read on with a dull curiosity. He learnt
that the name Sho, under its third form of Psa, occurs in an
early legend which describes how the deity, after the manner of
Jupiter on so many occasions, seduced a Virgin and begat a hero.
This hero, whose name is not essential to our existence, was, it
was said, the chief hero and Saviour of the Mesopotamian ethical
scheme. Then followed a paragraph giving other examples of such
heroes and Saviours being born of some profligate intercourse
between God and mortal.
And this is our natural reaction to such statements:
Then followed a paragraph--but Evan did
not understand it. He read it again and then again. Then he did
understand it. The glass fell in ringing fragments on to the
pavement, and Evan sprang over the barrier into the shop,
brandishing his stick.
The Catholic doesn't think about these things unless provoked, he knows that the Virgin Mary is his mother and recognizes that other religions have other mothers and leaves it at that.
The prophecy that "A virgin (or even a young woman) shall bear a son and he will be called God with Us" (Isaiah 7:14) may be a prophecy that is not unique as a prophecy to Christianity or Judaism. But it is unique as a matter of fact, in that it came true.
Christianity is about covenants and renewal, God's part of the covenant is that He keeps His promises. How He fulfills them is purely up to Him. Nobody expected wisemen to come looking for Jesus following a star, except maybe the wisemen and God. Catholics also believe that whereas Christianity is the fullness of truth, other religions have hints of it and sometimes more than hints.
That these things should happen throughout history is just a foreshadowing of the goodness of God which is written on our souls when we're created. It's unavoidable that things which are the same should bear some similarity, it doesn't mean everyone looks at them the same way or everyone is right about how they look at them.