Because if lay users can see how I vote, then I might start being harassed by people for daring to downvote them or daring to upvote someone. And may stay tracking my voting habits.
In which case, I’d probably stop voting.
Having a barrier to that info is better than no better even if it’s not impossible, imo.
People win suits against the police all the time. It’s just the police rarely face consequences for it, especially as an institution.