Because they don’t last. It’s the T Cell memory that does and they can’t test for that with the antibody test.
At least that’s my understanding of it!
Edit - doesn’t explain how she didn’t get it in the first instance though I guess!
Yeah, that's the thing I don't get either. And then I should be reinfected again considering that my antibodies dried off and she was probably infected 15 days ago. I doubt I had antbodies 15 days ago that I don't have today.