It's a very small universe to assume that only illegal activity is conducted with 10k in currency.
The argument for reporting is equivalent to "you have nothing to hide" and privacy. This is another aspect of privacy.
Furthermore, they don’t need the details. What if I just don’t want the government or bank to even know that I spent $10K at “Smucks Sex Store Co.”?
Anyway, I hope you’re starting to see the importance of privacy and cash. Even if they’re ridiculous and silly to someone else.