It's economically unviable to sail out with a fleet of recovery tugs to tow it back to a base, get a skeleton crew to repair it, have an escort to prevent it from being raided... etc etc. Not only that, but it might be a ship blown way off course and ends up lost, the crew went rogue and eventually starved to death etc.
There's plenty of reasons, but you really don't need anything more than a handwave ("It's there because I say so!") because people will be too busy going "HOT FUCKING DAMN, FREE SHIP!".
...Course, There's stopping you from trying to repair it up and take it as your own.