Presumably, very simply, because the formats are totally different and it is not possible to play the tapes on a VCR player with or without an adapter.
If it was as simple as an adapter, some third party producer would have made one by now.
The usual way forward would be to buy an old Hi-8 camcorder on eBay - preferably one with a DV Digital output like the DCR-TRV330E.
There was a VCR made for Video 8 tapes: -
http://www.ebay.co.uk/itm/Sony-EV-C500E-Hi8-Video-Recorder-very-rare-/221002303834?pt=UK_ConsumerEle...
:smileyconfused:.
