Why do aircraft fly over land rather than over the Adriatic Sea?
Most air routes in Europe are unchanged since the days before GPS, when they were defined by radials from VOR and NDB radio beacons. As these have a limited range, they could easily be used to define routes across the Adriatic, but less so down its several-hundred-mile length. Routes would therefore normally be defined over land where the plane can route from beacon to beacon, rather than over the sea where (unless you are flying towards or away from a coastline) you cannot fly with as much navigational precision as when following a radial to/from a beacon.