Either this isn’t happening because people wouldn’t accept that form of employment, or it isn’t happening because it’s too cumbersome or the cost of it cancels the gain. Or it isn’t happening because the laws are successful in preventing it. Who knows - either way it’s not happening.
Or it's not happening because people are too stupid to see the potential.
What exactly do you gain? The leave pay is paid by the government and you still need a new employee.
The only gain is that you can employ a new full time employee instead of hiring a temp while waiting for the person on leave to come back. It's a minor gain compared to the hassle/risk of trying to circumvent a law anyway.
Maybe for the most unskilled labor. Try to hire qualified professionals with those conditions and see how that goes.