This is where we see that most modern people, and therefore most Protestants, are not terribly familiar with your Bible.
If you read the Old Testament, it is very clear that God often appears to people or sends them messengers (angels, etc.) or dreams. This often happens in Genesis and Exodus, but it also happens in later historical books and in some of the prophetic books.
When this happens and the message is significant, usually the person in the OT thinks to himself, “I’m going to put up a rock or a marker or an altar, so that there’s a record of this.” (Jacob thinks, “
If this message from God pans out and I become prosperous,
then I will come back and build something here, and call the place Bethel.” That’s Jacob for ya. That’s probably also why he had to be wrestled.

)
David and Solomon building the Temple in Jerusalem follow this pattern, although David’s bright idea is actually temporarily nixed by God in a message. But the whole reason that David bought the threshing floor which would eventually become the site of the Temple? Because David saw the punishing angel that was bringing an epidemic to Jerusalem.
2 Samuel 24:16-17 –
“And the angel of the Lord was by the threshing floor of Araunah the Jebusite. And David said to the Lord, when he saw the angel striking the people…”
Occasionally, the Bible will not give you the person’s thoughts, but they’ll just say, “Oh, yeah, there’s a stone or a tree or some other landmark here, and that’s where it happened,” with the implication, “Be sure and remember to visit it if you go by there.”
But the third possibility is that God, or God’s messenger, will order specifically that the person build something at the place. Even the building of the Temple follows this pattern in part, since Moses was instructed in exactly how to build the tabernacle and the vessels.
When Mary (or another saint, or an angel) appears somewhere to somebody, Mary (or the other saint, or the angel) is acting as a messenger of the Lord. So it would only be natural for a person to whom Mary appears to think, “Yup, there ought to be a chapel or a church put up here.” And there are many cases where that is what people think, and they just ask the bishop to let them do it (or they go ahead and do something temporary, and then the bishop has to haul them back if they go too far, or approve it if it doesn’t claim anything too much). And indeed, that is what usually happens at approved apparition sites.
But if Mary appears to some humble person or a kid, who would never think of asking such a thing or starting to build a remembrance, she does usually nudge or order them to build something. Why? Because it is unfitting that God should send a message, and that people should forget it.
Also, it serves the practical purpose for Jesus (who set up the system of bishops, remember) of making sure the bishop comes into the picture early, and that the mortal persons involved with the apparition have to talk to him and obey him. (And the bishop has to pay attention to what’s going on.) Big prophecies and wonders usually are directed, in part, at making sure that God gets the attention of the people in charge! (It’s good for bishops to see that God is at work; and if bad stuff or imaginary stuff is going on, it’s good for bishops to catch it.)
I’ve got more to say in the next post!