Covid-19 - Why are there still a lot of international flights?
The simple answer is that no one has shut down any airspaces yet and that is what stops planes from moving. 

While many governments have put travel bans in place for people airplanes often carry other things like mail and cargo and it may still be viable and profitable to fly them. There is also the compounded fact that many people are trying to get home to quarantine them selves and in the short term there is still high demand for flights to move people back to their home countries. 

Airlines are starting to cut flights due to reduced demand and over the next few months you will see a drop in flight traffic but if the airspace is open planes will keep moving.