It would have to be a pretty novel (and likely much more complex, less reliable, more costly, etc.) design to attach to a thing that has multiple degrees of freedom as opposed to an immobile window sill with a built in clamping mechanism. Also worth noting that window-mounted a units often introduce some gaps in the insulation that the window would otherwise provide. In America, no biggie on that, we build with double pane windows primarily and many existing buildings even still have single pane windows. The net loss of insulation (if it is even a loss) is easy to justify/mitigate with some cheap treatments like foam inserts. For European triple pane windows, the loss of insulation is still not a huge deal (especially if you account for luften), but it is yet another factor that discourages market penetration of a hypothetical window mounting solution. Window units have been around for decades because they are successful at taking advantage of the nature of American windows. Because European windows and building construction have a fundamentally different nature, it's not surprising that this approach has not penetrated that market. It's not as simple as capitalizing on the original idea, it would essentially be coming up with an entirely novel idea that may or (more likely) may not be viable

I think the engineering would be so complex and so particular to a window type/size, that it's just not worth it. At some point, installing an external unit and wiring/ducting through the wall is actually the simpler solution.