Most of the airports on the continent are in EU member states and so they have to comply with Regulation 300/2008 on common rules in the field of civil aviation security (plus the subordinate regulations and the delegated regulations). This requires members to impose strict rules on boarding aircraft. And it goes one step further by requiring that the carriers draw up procedures and submit them for approval.
What this amounts to is that boarding a commercial flight in the EU is a controlled activity which takes place under regulatory supervision. If there is a breach of procedures, the carrier and the airport (and in big cases the member state) can be fined (that's a bad thing). If there's a grave breach, the carrier may be banned from EU destinations (that's a very bad thing).
So in order to have two separate boarding sites, the carrier would need to double their infrastructure at those sites (not to mention building them). Plus they would have to have a secondary procedure when two sites were in use and get it approved. If the carrier could satisfy the regulator with their procedures they could probably implement them. But it seems the carriers do not want to bear the extra burden of paperwork and infrastructure expense. Probably if one carrier goes through the rigmarole to have multiple boarding sites, then the others will follow by herd instinct. But the cost will be passed down the demand chain until it reaches YOU.
Some of the newer members may still be in the convergence stage (e.g., Hungary). There are 7 such states in total. When they get full membership their procedures will be compliant just like the existing member states.
Just to be complete, the procedures for boarding, getting off, transiting, deplaning, and whatever are all governed by the same regulatory framework.
This shouldn't be taken to mean that some carriers not have drawn up procedures, particularly the smaller craft that board from the apron anyway. Also, there will always be people who discount the influence of regulations as irrelevant bureaucracy, but in truth the body of airline regulations in the EU are actionable against carriers and airports alike. It is a ponderous corpus of important law and shouldn't be easily dismissed as irrelevant. Airlines are not "sovereign citizens who live outside the law", and almost every aspect of their operations is under regulator purview in one way or another. Carriers who opt to ignore them as irrelevant actually will get into trouble and face fines and other sanctions.
The alternative view is to say, There are no EU regulations that control how passengers board/deboard/etc. It's all done with by opening all the windows and doors and everyone jump out any which way they can. I'm in more of the first category, hence this answer.