I wondered the same thing about a year or so ago when a neighbor here in the western U.S. decided there was money to be made raising wine grapes. He imported a deep red variety from northern Germany that seemed to be suited to our climate and soil. Surprisingly, they did quite well here in cow country... so, I asked, as they ripened if they were edible. He invited me to try a few. Yuk, yuk, yok... well... they weren't that bad but entirely "un-grapey". They had lots of seeds... thick, tough skins and were very sweet. The new vinter explained the higher sugar content of the wine grape was required for fermentation.
I've not had the chance to try the white varieties, but the reds are not suitable for eating...