Salvage law doesn't entitle you to just automatically keep stuff. You're entitled to fair recompense (including a profit) for your work - and in the event that nobody will pay you can achieve that recompense by selling the thing salvaged but that's a bunch more effort
and you may have to fight in the courts to get/keep your money. These are specialist skills, why not earn slightly less money with far lower risk doing a normal on-contract at-sea recovery job?
Also, while today many countries think it's very important not to - for example - spill vast quantities of toxic materials into the ocean, this was not originally a concern of salvage law, and so not all jurisdictions will be friendly to the idea that you deserve reward partly for preventing an environmental disaster. Countries with modern salvage rules would pay more for the fact you're preventing a massive oil spill than if you "salvaged" millions of gallons of say mild saline solution that were inexplicably being moved and aren't toxic, but the work is the same even if you're paid less.