Because you can walk around any big city and find loads of girl more attractive than her. The bias of her being a pop star gives her a boost as well. Dont get me wrong, shes not ugly but shes no super model or Beyoncé.
In subreddit r/trueratediscussions
In a post titled:
Why is Billie Eilish considered mid or even unattractive according to some people when without her usual styling she has a very beautiful face and a good body?