Why? JA knows exactly where she'll be spending the rest of her life. It's no mystery to her or anyone else. Her showing, pretending, or having "interest" in any witness is not a requirement. That the public thinks it should be a requirement for her to do anything but sit there (quietly) is misplaced energy.
Remember Elizabeth Smart? Well in the trial for her crazy abductor (Brian Mitchell), he made a scene most days in court, sang hymns out loud, interrupted proceedings, and they had to put him in a separate room with a TV feed of the trial because they couldn't get him to be quiet in the courtroom.
I'd rather the trial proceed smoothly, which it is, and could care less what JA does or where she looks or if she's interested or not interested as long as she sits there quietly, doesn't disrupt the proceedings, and Juan and company can do their job.