Why does the Fat Lady never tell a passing professor that students were out of bed at night?

Question

Many times, the Fat Lady — the guardian of the Gryffindor Common Room — sees Harry and his friends going back to the Common Room after a late-night wandering, and she sees that they have been out when they are not supposed to.

So, why does the Fat Lady never tell a passing professor that students had been out of bed at night? She knows it's wrong, as she "tells off" Molly Weasley for being out at four in the morning. So why doesn't she tell professors about these out-of-bed students?

Note: I'm not a hundred percent sure about the Slytherin Common Room entrance and if it is able to communicate, but we don't know about Hufflepuff's entrance to their Common Room and Ravenclaw's knocker can only tell riddles (as strongly implied in DH, when Amycus Carrow tries to break in the Ravenclaw Common Room without answering the riddle correctly), so I've only included the Fat Lady in my question.

Answer 1

If awoken by a student, The Fat Lady's primary goal is probably to fall back asleep. She's been hanging on the wall for centuries- students out of bed is not a huge concern.

More importantly, portraits don't behave like living people. Like ghosts, they are 'imprints' of a painted person, explained in Rowling's entry on Hogwarts Portraits.

The portrait of the Fat Lady continues to indulge her love of good food, drink and tip-top security long after her living model passed away.

The Fat Lady is not concerned with maintaining general Hogwarts rules, unless they pertain to entering the Gryffindor common room. It is her sole responsibility. Molly Weasley refers to the portrait in Goblet of Fire, chapter 31:

"She was here in my time," said Mrs. Weasley. "She gave me such a telling off one night when I got back to the dormitory at four in the morning-"

So the Fat Lady becomes irate when woken, but won't leave her frame or bother with alerting staff, as that might compromise the room's security. She'd rather be asleep. It's possible that when the Fat Lady herself was alive, she wouldn't have cared if students were out of bed. Maybe that aspect of her personality is reflected in the painting.

Answer 2

The Fat Lady is the GUARDIAN of the Griffindor Common Room. She is a portrait that is heavily linked to that house. I doubt that she would betray the students of her own house and get them in trouble deliberately .

I suppose she might have still reported it if it was something serious.