The airlines are currently paying a "tax" by running the flights empty but that cost is just being burnt as fossil fuel and helps no one.
By switching to a tax for unused gates/ flight slots that is 80% of the cost of flying an empty if plane, the Airline saves money, the airport makes money, and the environment has fewer CO2 solutions. It is a win-win-win on all sides.
It helps everyone involved in fuel delivery, airport operations etc. Don't underestimate the government incentives to maintain the status quo.
If you switched this over to a tax you'd also need government programs to deal with the unemployment, lack of pilots and other ramp-up issues when this situation ends etc.
By switching to a tax for unused gates/ flight slots that is 80% of the cost of flying an empty if plane, the Airline saves money, the airport makes money, and the environment has fewer CO2 solutions. It is a win-win-win on all sides.