>>213023748
Could be for a lot of reasons, but they're an iconic part of her look. Maisie Williams is never going to be a conventionally attractive actress, so there's no real reason to alter what would otherwise be distinctive parts of her look.
Jennifer Grey was the 80s "it girl" for awhile until she got a nose job which made her nearly unrecognizable. In her words, "I went into the operating room a celebrity and came out anonymous." She never really got roles after that.
Same thing with Olivia Munn getting rid of her freckles.