Unfortunately, there doesn't appear to be any reason other than ripping people off.
I've always been wary of any product that claimed it would convert a signal with a simple adapter -- be it audio, video, or computer connections (hard drives, graphics, etc). This is a perfect example of why I suspect most devices of being misleading at best, fraud at worst. I think probably 8 out of every 10 adapters I've ever seen are either non-functional or only partially functional (noise, lost signal, dropouts, etc).
Computer graphics output has always been a messy situation because of all the available connectors, as is video output, thus creating a desire for conversion. However its conversion is generally no more technically possible than is converting a frog into a human prince.
-- otherwise excellent merchants for quite a few digital media and computer products -- sell a lot of this hucksterish crap on their sites. (In their defense, I think they often do not realize the items don't work, having been hornswoggled into buying it from the low-budget Asian tech markets. After the sales have come back to bite them in the ass -- losing them customers, gaining them bad reviews, and costing them returns -- their product pages fill up with warnings and disclaimers.)
I get rather tired of seeing, and fielding questions about, things like "USB to VGA adapters" or "composite to s-video adapters". No folks, it doesn't work reliably or fully. At best, the devices work with partial bandwidth, giving you a crap experience (example: intermittent video, choppiness). At worst, the devices are completely non-functional, like the example posted by Bix.
Thanks for sharing, *Bix*.
You've given me an idea for a new post -- a list of adapter devices that generally do not work.