I hope this question is relevent to Aviation. If not, please let me know. Below is a snapshot from Flightradar24. While there are far less flights than usual, there are still a lot of flights en-route. I thought there were controls put in place last week in several countries. Do these flights have special permission?
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.