It's not that she couldn't, it's that it just didn't come up (or perhaps she just doesn't have one). Well, this is Teen Vogue, and Greta's an up-and-coming actress and was asked about her opinions on fashion, as well, so I reckon that's why it made it in.

Personally, I love hearing little anecdotes like these from anyone who got to work on the Potter set, especially the actors - extras or speaking roles - so it's great.