Because only the fewest fields round here are public. Most are private, with severe limitations on their operating conditions. It is vital to operators to make sure visiting pilots respect both operating hours and local procedures. But even the public fields have very strict operating procedures, the circuit precisely defined with several VRP's. Check the EBAW VFR chart in the AIP for one example.
Europe is densely inhabited, in some places even very densely, it is impossible to fart here without offending one or the other neighbour. Much worse to fly an aircraft.
Disrespecting the given rules may not have much impact on the pilot, especially if he's a foreigner on a single visit, but may be all the worse for the a/d operator.
And you have no idea how stupid and stubborn some pilots can be. I remember well listening in to EBGB AFIS, a couple of years ago, when a chap with a heavy US'an accent cheerfully announced his arrival in an absolutely illegal way - the poor AFIS operator had to do all he could to get him into the official approach procedure, via LONDI. Had even to remind him he risked having his plane impounded if he continued as announced.