Well I can tell you with complete confidence that this is only because there are hardly any demos of the Osmose being used to control external synths full stop. And to make such a demo requires someone to choose the right synth for the job, and take the time to craft a suitable patch, and then take the time to play it nicely and record the demo. And whether it then convinces you will depend on personal taste as to what sort of sounds and playing techniques you think really make the most of the Osmose.
I answered this question with confidence because I've already got far enough with choosing a few synths and starting to mess around with patch design that I know I can get glorious results that elicit the same feeling of my fingers being connected to the sound via the Osmose as I get with some built in presets. But I am not close enough to refining such efforts into a state fit for demonstration, and then I will still lack confidence in my own ability to demonstrate this to other people, due to deficiencies in my own playing technique and ability to record video.
If this void of demonstrations of this type persists for too long then I will try to help rectify it, but I might need this to be a shared effort, eg I may craft the patches for a few synths and then try to find some other Osmose owner to do the playing and demonstration bit. Or some variation on this theme, eg if I can get some other Osmose player to share a recording of the midi MPE+ that comes out of their Osmose when they are playing a built in patch, and I then feed that into one of my synth patches and record the results.