Another reasons is that Congress, observing that patents and inventions are correlated, has made very effort to encourage patents so as to cause more inventions.
Don't forget that most people in Congress are lawyers. And lawyers will see the patent system providing a lot of value... because you typically need lawyers to file for a patent.
It's not necessarily a case of "keeping the patent system ensures jobs for lawyers." It's probably more a case of, "if you have a hammer, everything looks like a nail."
