I think that this is happening for 2 reasons:
1) The smaller indie developers can't afford publishers and retail versions.
2) The Germans (Daedalic, King Art Games), may have diffculties in finding english publishers, so in order to have the english version out in time, they publish a digital version.
If you take a look at the german Amazon (Amazon.de) you will realise that ANY (and I mean any) Adventure game gets a german retail version, even the crappiest one, including the non german-origin games! Not to mention the special editions, anniversary editions, game collections and so on.Germany is at the moment the land of Adventure gaming and the land of the Retail versions.