Why do I never see fighter jets on sites like FlighRadar24 and ADBSExchange.com
Because they don't want to be seen.


  Are fighters allowed to just turn off their transponders in civilian airspace?


FR24 and ADS-B Ex are not based on normal transponders (that would require access to actual RADAR data) but rather ADS-B and mode-S transponders. The military can simply choose not to operate with this equipment. This does not make the fighters invisible on normal ATC radars, since ATC obviously still needs to be able to see them.