Because the brother/sister relationship between James and Lily is adorable. This is over some holiday when all three of the kids are home, James has already graduated, Albus either graduated or is in his 7th year, and Lily is somewhere between 15 and 16.
"I'm going out," Lily announced, grabbing her jacket and heading towards the door.
"Not looking like that, you're not," James protested.
"Yes, I am."
"No."
Lily's hand was on the doorknob, but James grabbed his wand and flicked it at the door, so when Lily tried to turn it, it wouldn't budge and, as she wasn't 17 yet, she couldn't just say, "Alohamora."
"Albus?"
Albus shook his head. "I agree with James," he said.
So Lily left the house wearing a knee-length black dress with a high neckline, and the original short skirt and low-cut shirt hidden in her purse.
~writergal24