So the cost of registering your work
https://www.copyright.gov/docs/fees.html looks awfully expensive, usually much more than people do commisions for, and how much of the fees and effort, time included did you recoup from those people you spent your invaluable time to chase down?
Sounds like Watts and his steam engine patent, until he held the patent he didn't innovate or do anything else besides fighting to uphold his monopoly.
Copyright automatically applies to any photo I take. I don't need to register it. Licencing of images can be 1000's of dollars per image depending on the use so it's worth chasing people if they've taken it without consent. It doesn't happen often but even once is too much.