They do it because they get to keep the seized money. Generally speaking, it will get divvied up between the prosecutor’s office and the reporting agency office and added to their operating budget. Individual agents may also get a bonus or it is used to bump their employment record and qualify them for higher ratings (= higher pay).
Somebody at the bank (a teller or manager) also reported them and received a “reward” for doing so.
This is happening to small businesses all over the US that receive significant cash payments that must be deposited frequently.
You can thank our dimwitted Congress and the War On Drugs for this unconstitutional institutionalized theft.
They should have shot Wickard, and sent Philburn to Congress.