Who says there isn't? There are some devices to use Nikon/Canon lenses with a DB. Image circle isn't always sufficient though. Horseman also make conversion lenses. Was a time when conversion lenses were used as reporting aids by a lot of broadcast video stations.
Just because you and I don't like something doesn't mean it doesn't exist.
That Komura link is for front-of-lens converters. Sure they exist, and Horseman's ones too. I was saying that what does not exist, to my knowledge, is any "rear-of-lens-adapter-with-glass" to make a 35mm format lens reach infinity focus on a MF SLR. And that was in response to liac's "if there are M42 to Nikon converters with glass, why not for MF?" question. S/he had swung the thread away from front converters to a different thing entirely.
Yes as you say, "There are some devices to use Nikon/Canon lenses with a DB" - but that is not with a bulky MF SLR body, like a Hasselblad, interposed between lens and MFDB. Liac's heart seems to be set on a Hassy body. I'm just trying to answer his/her questions as factually as possible. There may be an uncomfortable collision between his/her dream and the reality of available photographic equipment, but we have to be honest about it.