Boredom
A small black portal appears in the middle of the cafe and out of it flies Susan. Landing on a table she seems to ponder for a moment but then with a shrug of her tiny shoulders she pulls one of the napkin holders over on its side and uses it as a seat and remains on the top of the table rather than take a human form and sitting in a chair.
After all one of the major pleasures of the cafe was the atmosphere that allowed one to be one's self.
Rather than disturb the overworked cafe staff Susan opens a portal and reaching in pulls out a drink and slowly sips it while watching the patrons of the cafe going about their business. She really hoped someone she knew came by so she could have a chat. She was feeling restless and bored.