I'm gong to say "Because they really look nothing alike" - May as well as why they haven't mistaken her for a particularly tall dwarf or a much less draconic dragonborn, the answer would be the same.
Then how about mistaken her for a tall goblin? Someone drunk? Maybe? Saying "oh, a half elf half goblin".
The hobgoblin iron shadow in Volo's Guide is a female hobgoblin, but wearing a devil mask. They seem to look much like the males, but with more humanoid feminine features. Considering the same is true for goblins, I'd think the same would also be true for bugbears.
Must be some very pretty face under that mask. Hmmm, must be.